Similar to this deal, but a bit cheaper: https://www.ozbargain.com.au/node/514244
works out to $14.85/month, $16.50/month or $23.20/month
Lebara uses the Vodafone network
Last year you were able to buy and stack 2 of the 180 day cards. Not sure if you can still do it.
I've been with them a couple years and never had any issues.
[edit]: Their online chat confirmed that you can buy up to 24 months worth of plans, they will run one after the other, just buy them from their website. They also said its only for existing customers.
[edit2]: expiry changed from 12th to 31st May.
How does the stacking work? And is this for existing customers only?