The greatest task in Australia is trying to find some uniquely Aussie food to give guests from abroad. While seafood at the Sydney Fish Market is good, there are actually numerous great seafood restaurants around Sydney. Many are little hole in the wall places where you order a platter including lobster and crab.
So seafood platters and fisherman's baskets are pretty cool. The quality and amount vary based on how much you pay.
Let's not discount eating fish and chips on the beach, especially at Bondi. Sydney does some amazing quality fish & chips at a great price.
The other option are pies at Harry's Cafe de Wheels. They now have a branch in Haymarket as the one at Woolloomooloo was a bit out of the way.
The other dining experience in Sydney is to go to a eating street where you walk down the road and decide which country you are going to eat in tonight.
A final thing to try is during certain events, especially Opera in the Park and Symphony under the stars, the food stalls are operated by 5 star hotels. So you sit on a blanket with 100,000 other people, listen to cultured music and enjoy food from the top hotels in Sydney.