Best iPad Data Plan for Mum and Dad (Pre-Paid with Easy Top up, 500MB-1GB a Month?)

Hi guys,

To get my parents online I bought them 3G iPads and TPG $1 Pay as you Go SIMs (given their likely very low usage).

Years later great news is they have really gotten into web surfing, booking hotels (without me doing it for them!) and I can Facetime and email my parents often.

Down side is their usage has shot up and TPG seems to have discontinued the amazing value $1 SIM plan. I keep getting $20-60 monthly bills now (each!).

What would be a good option for them? 500-1GB per month roughly, pre-paid credit with longish expiry type plans? (they are not wealthy and would need to carefully manage their recharges) - Mum has WiFi at home, Dad not.

Optus network at least maybe Telstra (Aldi/Boost) (no Voda).

Thanks!