If you have an Access Card on your account, you can deposit cheques1 and cash at any Australia Post outlet displaying the Bank@PostTM sign.
You don't need to complete any forms, simply present the cheque(s) and/or cash at the Australia Post outlet. They will ask you to insert your card into the machine, select your account and enter your PIN. The funds will go into your account straight away however will not be accessible until the cheque is cleared.
1 Cheques need to be payable to yourself and must match the name on your Access Card for Australia Post to accept them.