Australia Post is putting up their letter pricing tomorrow. The basic stamp goes up from $1.20 to $1.50. There is no use on stocking up on stamps, as from tomorrow, you would need to use more stamps.
But you can stack up on prepaid envelopes - either normal prepaid envelopes, or the green domestic letter with tracking envelopes. Both these types of envelopes are going up 20-25% tomorrow. They don't expire.
You can buy on the Australia Post website or through their eBay (Topcashback has 3% cashback).
More expensive to post something a few street over than it is to have something small shipped from china using AliExpress.