Before you sign a sales contract, be it an Auction, Off the plan, or House and Land, it is always a good idea to obtain a Pre Approval.
Pre Approval letter. For a start, you will not have to waste a lot of time looking at properties outside your price range. You will also feel a lot more confident at bargaining the sale price with the vendor or agent if you know exactly what you can afford. If you are buying at an auction, you have a good idea of the maximum bidding price that you can bid at.
The actual lodgement of your loan should be made 1-3 months prior to property settlement. Each lender has a dyamic turnaround time. You should choose a lender with a turnaround time that will meet your needs.