Lenders also factor in an allowance for interest rates to increase when determining your affordability. As a result, what's positively geared for you, probably won't be positively geared from the banks perspective. You can hit a serviceability wall with the banks even if you are positive cash flow.
From the lenders perspective, the exact figures vary from one lender to another, but as a basic rule, assume you need about 3% higher than current rates just to 'break even' on serviceability. If rates start to increase, this margin will probably go up too.
This is a rule of thumb, you shouldn't entirely rely on it, get proper advice from a broker or lender.