Keep a base case you can explain
Start with the rent you expect, the costs you know and the loan you are considering. Save that scenario before changing assumptions. It gives you a reference point for each tougher case.
You do not need to predict the next interest-rate decision. Find the cash requirement under conditions you would want to be able to handle.
Illustrative shocks to explore, not recommended settings or predictions.
Combine the changes
Increase the mortgage rate, add vacancy beyond the normal allowance and enter a one-off repair. Lost rent may reduce percentage-based management fees, but rates, insurance and many other bills continue.
The matrix compares annual cash flow at several rate and vacancy combinations. The separate combined test also includes the repair amount. You can see the cost of an ordinary year beside a year with a large bill.
Put a cash buffer beside the shortfall
If stressed annual cash flow is negative $18,000 and you have $20,000 available, funding that annual deficit would leave $2,000. That does not tell you whether every bill can be paid on its due date; it tells you how much of the reserve the year consumes.
Money in offset is accessible, but drawing it down can increase interest. This calculator holds the offset constant, so run a lower-offset case if you expect to use it.
Decide what would change your decision
If the combined shortfall is too large, consider a lower purchase price, less borrowing, more available cash, or another property. Increasing rent in a model is useful only if the local market can support it.
The result is a budget test, not a probability score. Save more than one scenario if it helps you distinguish a manageable setback from a purchase that would stretch your finances too far.
Further reading: ASIC Moneysmart: property investment.