It really is what you make of it.
A lot of people have at least some people they know from highschool and they tend to stick around these people quite a bit.
I've noticed this especially at monash, a lot of the kids seem to come from eastern suburbs highschools and already know heaps of people by default.
All great for them but i'm from the west and no one i knew in HS went there, so it wasn't quite as easy as having a "ready made" social network.
Most of the time the people you meet in the first couple of weeks of both semesters are people you'll likely to see more of if you keep it going. Tutes/Labs are often smaller in size (eg 10 to a lab bench or tutes of like...20 in arts), so thats another good way to get some social interaction. In the really huge courses though, like science, lectures and subjects are often packed. My bio lecture had...1000 students...just in the one lecture. So, its a bit hard when you're in a sea of people sometimes. You could go all year and not see the same person twice.
Its hard to meet people within lectures themselves unless you want to talk during the lecture but you'll probably get the vibe you really shouldn't.
Tutes/Labs offer a better alternative. My social life in my course this year was pretty crappy but i realise i wasn't really doing heaps about it either (partially because i lived like 2 hours away and it limited a lot of things). You really have to put yourself out there, it is what you make of it. Talk to all the people in your tutes/lab bench and i'm sure you'll find at least some people. Even though i didn't do all that well i still managed to meet a few people, one of them for example i met just talking to outside the lecture theater while we were waiting for it to open and took it from there.
So, i guess if you're worried my main tip is just talk to as many people as you can, don't be shy or worry people usually don't bite. It'll get even worse and worse if you put it off. Clubs (supposedly) are pretty good for meeting people as well, so join a couple of those if you can. I guess you can also try talking to international students which a lot of people i see tend not to do o a certain extent, i guess because they think they'd rather stick to themselves or it'd be weird or something but most likely they won't know a shitload of people and will probably be happy to know someone.
But yeah, it can be hard sometimes in a course as huge as science. Long as you try you shouldn't do *too* badly.