Good on you for thinking about this - there aren't enough rescues in Australia. I'm also from Aus - I currently have 14 pigs, all were unwanted from various places (private homes, vets, shelters, etc.) and have a permanent home with me here. Just a few thoughts from me:
I'd recommend speaking with other small rescues in Aus - they'll be able to give you an idea of the costs and other considerations, including legalities, involved. I found a few in NSW with just a quick Google search.
I also think $900 is a little low to start - it costs me A$90-110 per week for my current 14 on food (hay + veg + grain) alone. You also need to factor in electricity, bedding, vitamin supplements, equipment, shampoos, etc. etc. etc. I average A$1100 per pig per year. This is with super-careful planning and budgeting.
And of course there's the vet expenses. Nearly every one of my pigs came in with something 'wrong' - including serious stuff like scurvy, malocclusion, severe mites, bumblefoot, stasis/bloat, etc. Finding a committed cavy-savvy vet in Australia can be difficult. I'm fortunate that I'm a nurse and can do a bit of the basic vet care myself. A lot of my guys have ongoing health and behavioural issues too - they could never be rehomed to a regular home (not that I do this).
You also need to think about the time involved - my pigs take 60-90 minutes per day in cleaning, feeding and other basic husbandry chores. Caring for a critically ill pig can be a 24/7 job for a number of weeks. And I imagine having a proper rescue would come with other tasks - fundraising, dealing with owners, etc. It's a constant juggle with work and there are no days off!
I never had any desire to set up as a rescue, but one thing I did find was that when people in your community know you 'take in' (or even just 'have') guinea pigs, it's very easy to become overwhelmed with the number of unwanted pets. It can be heartbreaking to say 'no'. A few years ago, I met a lady running a 'small' rescue in Brisbane who, through surrenders, dumpings and pregnancies, ended up with >120 pigs. I don't let anyone except a few trusted vets know I keep pigs!
As I said, good on you, but go in with your eyes open.