If you want to see dophins the best place for me was a place called Tin Can Bay near Rainbow Beach & not far from Noosa which is also a lovely beach town. (All on the Sunshine Coast)
In Tin Can Bay you can actually stand in the water & feed fish (which they supply) to 2 dophins that regularly come in. It's a wonderful experience & they don't even charge you as it's all done voluntarily. (You can make a donation if you like which I did happily)
A bit north of there went on a trip to Lady Musgrave Island from the Town of 1770. It's one of the lowest points you can go to the Barrier Reef from. It was really well run. When you landed on the platform and after your visit to the island you could snorkel to your hearts content, scuba dive, go out on the glass bottomed boat or just enjoy the wonderful views.
In South island NZ take the helicopter trip to the top of the glaciers. It costs a bit but was one of the highlights of my trip to NZ. Milford Sound is a must also.
Just back from a wonderful trip of NZ & Oz and they were my 3 highlights.
By the way I loved Perth & Fremantle (so easy to get around )& thought that once you did the Harbour & the Opera House & the Rocks in Sydney nothing is left to do there. Melbourne has loads of things. The MCG - go view the real Aussie Rules for a laugh or the Cricket - the Old Melbourne Jail of Ned Kelly fame, go down the St Kilda for old world seaside fun, swim with dolphins in Sorrento, drive out the Gt Ocean Road ( I've been to Melbourne 3 times & never ran out of things to do whereas 3 days in Sydney was enough for me !)
My point is that if you ask 10 people they'll give you different answers so you really have to read up & decide for yourself. You'll have a wonderful time anyway !