Found a possible hack when purchasing a new Vodafone plan. Purchased the highest plan on the 18th, then noticed my billing cycle resets (and I get billed) on the 19th.
Contacted to cancel 'at the end of my billing cycle' as I only wanted 1 month, they ended up immediately cancelling and no charges at all.
All in all, I had access to 800GB of data (student discount), used ~100GB data to download what I needed to, and paid nothing.
YMMV, use method at your own risk and be prepared to potentially pay the minimum charge (1 months cost).

Vodafone have a 30-day "network guarantee"
https://www.vodafone.com.au/network
They also run a credit check when you sign up.