Yeah, ya duffus. How are we supposed to plan an itinerary? Dates.
You sound like you have all the right information, just not real confident about it.
What's the Cheapest way to travel? What's cheap these days? Trains and buses. Here in Melbourne there's a tram that circles through the city that's free. It does laps, so expect a wait. I'd just walk around. There's the Botanical gardens alongside the Yarra river. Melbourne Gaol where they hung Ned Kelly 130 years ago. The zoo, the aquarium, the museum, which backs onto an IMax. The Exhibition buildings might have something on. Plenty of pubs, live music, theatre, different cuisines. You might want to take a train to Belgrave, which takes you to Puffing Billy for a trip through the hills, onward over the Dandenong Ranges mountain, for a walk through the bush, you might be lucky to hear a Lyre bird, but it can be hard to tell because they're such good mimmickers. Wait for the chainsaw, thats a giveaway. There's an artist retreat there, an old bloke, who died not long ago, sort of a hermit. Walk through his garden and check out his sculptures. Over the mountain you might want to trek through the wine valleys, on your way to Healesville Wildlife Sanctuary.
Uluru is smack in the middle. Get in, get out. I think they still allow tourists to climb The Rock, but there's been steps made to try and stop it. Never been there myself, so I should shut up. Been to Brisbane, took in the Amusement Parks, Wet n Wild, WB Studios, Dreamworld and Sea World. Stayed in Surfers, Paradise that is, and enjoyed Ripley's Believe it or Not, the beaches, the tropical rain forest, animal sanctuaries etc. Tasmania is cool. I mean it's really cool, bring a jumper. Lots of different scenery all the time. Hobart is small, a ten minute drive and your deep in the country. Huon Valley has a Jet boat ride. A lot of history wherever you go really. Port Arthur is the site of the first penal colony, and there's some ruins which are particulary interesting. I went to one of those 'walk through the trees' places. A suspension walk through the upper canopy. There's a few caves to check out.
Where was I? Walk ya lazy coot, busses and trains.
New Zealand isn't far. You can check flights online to know how much and if it's within your budget. You'd want to spend two or three weeks, and you'd be rushing it even in a hire car.
Hotels can be expensive. Hostels sound like your best option. They're everyhwere, but funnily enough I never notice them.
Food. Get a counter meal at a pub. Be careful because the same meal can cost you more in the bistro, order at the bar.
So, all of this being said, what you need to know depends on how long you have. If you want to swim, go to Brisbane and head to Surfers, where the water's warm all the time. It's bloody bracing down south, but it can get hot, then it's just what you might need.
So what are a you coming here for exactly? And how long? It's the sex isn't it? Just prop in Sydney.
I'm more interested in the cities, the nature, and the food. I've considered going to Hobart, I think adding a flight within Australia (and Tasmania) is only $100, so I figured why the hell not? So that's definitely a possibility. New Zealand would be a quick trip, but like I said, I'd definitely be going back there for another vacation. I need to give New Zealand the attention it deserves. 
