Most contracts have a clause that says that notices that need to be given under a contract will be deemed to be received, if posted and received in Australia, after three days. However, Australia Post has changed all that.
If you put a regular (currently $1) stamp on a letter for domestic delivery, then Australia Post says it will be delivered within 2 to 6 business days from the day after posting.
Note that Australia Post says "Delivery times apply from the day after posting".
If you add 50 cents you can send a Priority letter which should be delivered 1 to 4 days from the day after posting.
Action you need to take: Do your contracts allow sufficient time for delivery of postal notices? If you need to respond within a very short timeframe you don't want to lose some or even all of that time because you simply haven't received it. You may want to specify that notices must be sent at least by Priority letter, and deem receipt 4 days from the day after posting. You might even require a faster service option such as email, but make sure that clause is properly drafted as well.