You can go to this website http://www.swimmingpoolregister.nsw.gov.au/ and register your pool. At this stage your compliance certificate can be obtained through Council. Soon other companies that are certified to give the certificate will be available.
Depending on the circumstances and as to what you are actually purchasing this can be required at different stages. However, generally it is 3 months from the date of exchange or settlement, whichever is the earlier.