Melbourne is a Major city of something like 5 million people.
There is not really much in the way of "camp sites"
and any Caravan park in Melbourne I wouldn't call camping, most are cabins and caravans
You will need to get out of the city, Country trains are reasonably priced
Do you have any Idea which direction you would prefer to travel ?
eg, North, East or West (south will get you in the bay lol)
I live 230 km West
of Melbourne in a town called Stawell
Stawell is only 20 minutes from the Grampians National Park ( Train and bus from Mlebourne)
It's a very beautiful National park and camping fees are about $13 per night per site (no reservations)
Halls Gap is the main tourist town there with a few Caravan Parks. (eg http://www.hallsgapcaravanpark.com.au/
Some nice photos of Grampians herehttp://www.yktravelphoto.com/places/gra ... rd/1788/en
Many National parks, State Parks have free camping areas
State Forests also have free camping areas.
But not always near public transport.
Check out http://www.ozcamps.net
for some good free / low cost camps
Cheap tent ?
If you are after a basic 2 - 3 person dome tent, a new one can cost you as little as $30 - $50
or as much as hundreds of $$$
Examplehttp://www.raysoutdoors.com.au/online-s ... escription
If you can narrow it down to a preferred area ( beach, mountains, rivers, lakes, full of tourists or peaceful bush)
you will get more / better advice