If you are buying at auction you must have all of your due diligence finished before the auction day. You should obtain a strata report and building reports if required. If you are the successful bidder on auction day you do not have a cooling off period and will need to sign the contract on the day and pay your deposit.
In terms of finance you should make sure that you have loan approval for the amount required because if you are the successful bidder and sign the contract on the day you cannot then change your mind and withdraw from the contract at a later date.
In terms of the contract you should always have a full written advice prepared by your solicitor at eConveyancing NSW and ensure you give your solicitor enough time to negotiate any changes to the contract prior to auction.