That's one of the many advantages of dealing with a well informed mortgage broker. They know and have access to the serviceability models of the various banks. It's not all about living expenses, either.
For example, some lenders are using an assesment rate of 8.5% at the moment (ING for example), whereas others are using P&I repayments to assess serviceability, even though you are applying for an IO loan.
Also, there is variation depending on personal circumstances, too. FOr example, one bank might have a great serviceability model for singles, yet a poor model for couples with dependants, so depending on your circumstances, you really do have to explore he options and not just rely one someone telling you that bank X has "the best" serviceability model.