I found out all about "test" interest rates - rates that banks use to test the affordability of your loan - when I applied for my most recent property investment loan to subdivide and build two houses. I only found out because I used a mortgage broker rather than hitting up the banks myself.
Banks set their own rates for this and don't release them to the public. It would definitely be useful for investors to know what these test rates are to ensure that they give themselves the best chance to secure the lending they're looking for.
Different banks have different rates. To find out more, have a read of this article by The Press.
Landlords bail on rental market
January 11, 2018
The "secret" mortgage rates banks don't want you to know about