It really really depends on what kind of holiday you want.
If you want a beach holiday- Queensland- Gold Coast/Sunshine Coast. It also has themeparks and rainforest and the barrier reef. I went there when I was 10 or less so I don’t personally remember too much of it except for that.
If you want beach and city and a few other things- Sydney. The Taronga zoo at Sydney is awesome. Manly is great, as someone else said, and the ferries are really convenient and a nice way to see the city. You could do the harbour bridge climb, depending on your budget. And I’ve just been up the Sydney tower for an all you can eat buffet lunch in the revolving restaurant- it was awesome. Even on a low budget, the initial price might seem big (I think it was like 50 bucks a person) but it is honestly allll you can eat, you can go back as many times as you like and it is well worth it, because you also get an awesome view. I’m not sure when it ends but the Biennale festival is on at the moment- that’s an art festival, and you get a guide and do a great tour of Sydney while also seeing some awesome artworks- and there’s a free ferry out to Cocaktoo island which I think is an abandoned metal works or something- pretty eery. The Powerhouse museum is great, I could spend a full day there. And the other museum is pretty fun too. The Botanical gardens are gorgeous. I enjoy Sydney more than Melbourne but there are great things in Melbourne too and the flight is only an hour from sydney and if you look online you can get really cheap flights (I got one for 28 bucks).
In Melbourne there’s the Eureka Tower with the viewing deck that goes transparent as you walk on it so you can see directly underneath you, there’s the National Gallery and the Museum, there’s the ACMI (Australia Centre for the Moving Image I think) which always has awesome exhibitions on, I think a Tim Burton one starts soon. There’s an awesome cabaret restaurant called Dracula’s, and there’ll always be things on at the Regent Theatre and elsewhere- here’s a secret: there’s a place called “Halftix” just near the town hall in Swanston St where they sell off whatevers left of that nights tickets to various shows (comedy festivals, musicals, draculas etc) during the day for half price. The trams are really easy- just a get a day tripper ticket and you can hop on and off anywhere- and use the same ticket for the trains as well. It’s nice to catch a tram to St Kilda beach and get dinner, Luna Park is there too with I think the oldest rollercoaster in Australia or the southern hemisphere maybe. If you want to spend a day trip- I’m not sure if there’s a train or bus because last time my family hired a car but the gold mining town at Ballarat is really good fun. You can search for you own gold and everyone there is dressed in colonial style clothes from back in the gold rush days and the shops are all from there and you can go down in to a mine and watch shows and marches and gold smelting.
If you really want to see Australia you could go inland- Alice Springs, and see Uluru etc, or go to Darwin and see crocodiles and that sort of thing. Also apparently Broome in West Australia is lovely, though I’ve never been over that side except for connecting flights.
Now, if you were interested in coming to Tasmania you might have to PM me otherwise this response could rack up a very high word count.
About New Zealand, removing any Aussie vs Kiwi bias, I don’t think that you could do Australia or New Zealand any justice if you were trying to fit both into two weeks. You can get fairly good priced flights but it’s just…. Auckland is just like Sydney and to do anything else- like gorgeous nature treks or something on the South Island you would just need way more time.
I know you say you want to visit as many places as possible- but maybe it’s better to visit less places and get more out of them, get your moneys worth, your trips worth. You may spend all your money and rush to every place then get home and feel as though you haven’t been anywhere at all, because you never got to appreciate one place enough.