1. To use the tool, enter the total change in the level of Commonwealth borrowing, in million-dollar units, for the relevant financial year(s).

• Users can only enter amounts between -1,000 and 1,000 for each year, representing changes in borrowings of up to $1,000 million.
• A positive number reflects an expectation that, relative to the baseline level of borrowing, the Government would borrow more in that particular financial year - this would generally cover where a policy proposal is expected to worsen the headline cash balance in that particular year.
• A negative number reflects an expectation that, relative to the baseline level of borrowing, the Government would borrow less - this would generally occur where a policy proposal is expected to improve the headline cash balance in that year. Note that this does not represent repayments of previous borrowings.

2. The default long-term nominal interest rate for the 10-year bond is currently 4.80%.

• However, your long-term nominal interest rate must be between 0 and 10.

1. While the Parliamentary Budget Office (PBO) makes every effort to ensure that the calculator is useful and up to date, the results produced from the calculator do not constitute the provision of professional advice from the PBO. The PBO is not responsible for how the tool and its results are used.
2. The calculator is a stylised version of how public debt interest is calculated and applies a number of simplifying assumptions (see the 'Detailed calculations' tab from the downloadable spreadsheet for more information). It does not reflect the complexities of the Government's debt management strategy, including decisions around instrument selection and issuance.
3. The calculator is designed for educational purposes and provides estimates for public debt interest impacts resulting from increases or decreases in Commonwealth borrowing compared to a baseline. Since the calculator assumes changes in borrowing are refinanced indefinitely, it should not be used to simulate scenarios where an initial level of borrowing is repaid in a subsequent year.
4. This calculator assumes that changes in borrowing are financed by the issuance of 10-year Treasury bonds. In practice, the Australian Office of Financial Management (AOFM) issues a variety of bonds with differing maturity dates to finance Government debt, so this assumption is less reasonable for larger amounts of borrowing. Therefore, the calculator should not be used to estimate public debt interest for aggregate debt levels.
5. Future interest rates over the period to 2033-34 remain highly uncertain due to various domestic and global pressures. In particular, the impacts of potential fiscal and monetary policy could have significant implications for Government bond interest rates and hence affect public debt interest estimates produced by the calculator.
6. While the calculator can be used to estimate the likely borrowing costs associated with a specific policy, it would not be appropriate to attempt to use it to calculate interest costs associated with aggregate debt levels.

PDI is the borrowing costs of the Government, mainly incurred through issuing and servicing Government debt, and recorded as a cost to the Government in the Budget.

PDI impacts estimated in the calculator reflect the interest costs associated with the change in borrowings that are input by the user. These are calculated using the projected interest rates for future borrowings.

PDI is an important element when considering the overall cost of new government policies, called 'measures', and their impact on the Budget. Although each measure that has a financial impact will result in a change to PDI, the long-standing convention in Budget Paper No.2 is that the PDI is not attributed to any individual policy measure.

In some cases, the interest the Government pays (or forgoes) to secure the funding can be the largest underlying cash balance cost associated with the policy. For this reason, it is standard practice for the Parliamentary Budget Office to include the estimated public debt interest impact in costings when financial assets are purchased or sold. This calculator allows users to estimate the public debt interest impact for costings that don't necessarily involve purchasing or selling financial assets.

Bond

Bonds are investment securities where an investor lends money to a borrower (typically a company or a government) for an agreed period of time, generally in exchange for regular interest payments and the repayment of the amount lent (principal). Bonds are a form of debt - an entitlement to receive or an obligation to pay fixed or varied interest payments over a set period of time. Bonds are a means for companies or governments to raise funds to finance ongoing operations, new projects, or acquisitions. Bonds have maturity dates at which point the principal amount must be paid back in full or risk defaulting.

The maturity date is set when the bond is issued. Since the calculator is estimating future debt, we make assumptions on the type of bond that will be used. The PDI Calculator assumes all debt is financed by 10-year Treasury bonds.

For more information about the bonds and maturities issued by the Commonwealth Government, see the 'Debt Statement' in the Budget Paper No. 1 of each Budget and the Mid-Year Economic and Fiscal Outlook. Budgets are available at https://budget.gov.au/index.html.

Government Debt

The Commonwealth Government finances its activities either through receipts or borrowing. When receipts fall short of payments, the Government borrows by issuing government bonds (known as Australian Government Securities (AGS)) to fund its operations and spending. The amount of money that the Government owes its lenders at a particular point in time is known as government debt. In broad terms, it measures how much successive governments have spent over the receipts they have collected.

The Australian Office of Financial Management (AOFM) is responsible for issuing AGS and managing the Government's financing activities.

Why are the Commonwealth borrowing amounts limited to between -$1 billion and $1 billion per year?

In reality, Governments sell a variety of bonds to finance their debt, not just one type of bond as the model assumes. For individual policy proposals with smaller financial impacts, this assumption is more valid, however it is less reasonable to assume large amounts of borrowings are being financed by the one type of bond. Therefore, the calculator restricts the borrowing amounts to a limited range.

For a one-off change in borrowing in 2023-24, why does the PDI impact jump up in 2033-34?

The PDI Calculator assumes that the nominal interest rate is fixed from the day the bond is issued to when it matures. For instance, if a 10-year bond is issued in 2023-24 with a 3.33% nominal interest rate, the same rate is applied each year until the bond matures in 10 years' time, in 2033-34. At this point, it is refinanced with another 10-year bond with its associated nominal interest rate for 2033-34 (i.e., 4.8% based on the yield path assumptions from the 2023-24 Budget). This results in a significant increase in the PDI impacts in the last year when it is refinanced at a much higher rate.

Why doesn't the PDI impact go to zero when I borrow in one year and pay back the same amount in a subsequent year, even after accounting for capitalised interest?

The PDI Calculator should not be used to simulate the impact of an increase in borrowing in one year and the subsequent repayment of that borrowing in another year. Any change in borrowing (increase or decrease) is a change relative to a baseline level of borrowing in that year, and has no link to changes in borrowing in previous or future years. Any reduction in Commonwealth borrowing in one year should not be interpreted as a repayment of previous borrowing.

The PDI impact resulting from a change in borrowing reflects the interest costs based on the interest rate for the year the change in borrowing occurred and this may be different from the interest rate of a previous borrowing.

For example, say borrowing increased by $100 million in 2026-27 and then decreased by $103.33 million in 2027-28 (accounting for capitalised interest of $3.33 million from 2026-27). The calculator treats the $103.33 million as a separate change-in-borrowing event with its own interest rate, rather than a repayment. As the projected interest rates for 2026-27 and 2027-28 are different, the PDI impact estimated by the calculator is non-zero (reflecting the difference in interest rates between the two years).

Note that if the increase and decrease in borrowing occurred in years where the projected interest rates are the same, the PDI impacts would perfectly offset each other, resulting in a zero PDI impact.

The interest rate profile for the period to 2033-34 can be found in the 'Detailed Calculations' tab.

Why does the PDI calculator only go out for 11 years when bonds issued by the government can extend beyond the 11 years presented here?

Consistent with the PBO's written costing advice for policy proposals, the calculator estimates the PDI impacts for the current Budget year and the following ten years.