MEL isn't an easy place to get to for spotting without a car, the main viewing areas aren't on a bus route, so ya can't get off along the way. Getting to the Airport terminal from melbourne your looking at a $50 + taxi ride, or $16 Skybus goes from Southern Cross train station in the city to the airport each 10 min. Or you get a train to Sunbury and then a local bus to the aiport, but that trip is not a regular trip maybe one an hr.
If you have a look at a map of the airport, there's two public viewing area's, to the North on Sunbury - Bulla Rd at the roundabout, if you go alittle though past that you come to an old church on Uniting Lane, thats where you're find the true spotters. The other is over on the west near the tower on Operations Rd, on the way to that one near an airport Gate 22 onwards is a good spots, but near Gate 22 you will be at the Southern end of the 34/16 runway. If you look at a map of Operations Rd you will see that you can get near a couple of Hanger areas, were there's always something sitting outside. Your best bet from walking from the terminal area for viewing the 27/09 runway, as it isn't far around to the North, on the road that takes you to the first viewing area I mentioned, you'll see the lead in lights.
Ya need a car Mate.