You can renew your Pass online before it expires or up to 8 weeks after it expires to be eligible for a renew discount. More than 8 weeks after it expires you are no longer legible for the renewable price and you will need to purchase new annual a passes.
You can renew you Merlin Annual Pass using this link. Tickets & Passes | SEA LIFE Sydney Aquarium be sure select a renewal ticket option, and have your current passes ready as you will need to input this information.