You will need to register at One Big Switch (as per link) and you will have the option of buying either of the following amounts. There does not seem to be a definite quantity on how many you can buy, however I'm sure there will be limited gift cards.
Pay $50 - Get $53 worth of value.
Pay $500 - Get $530 worth of value.
*Works like an EFTPOS debit card
*Valid for all fuel types including diesel, plus in-store products
*Valid at participating BP outlets with eftpos Australia-wide for 12 months
This deal certainly beats the petrol vouchers the major supermarkets are offering, however the obvious drawback is you will need to pre-purchase the amounts.
I usually use the Caltex gift cards (Wishcard) which are sold at 5% discount at RACV (need membership … but there are more ways to get these Wishcards with 5% discount) … then combine it with 4cent vouchers (when possible) … comes at a similar saving …