An actual deal at an actual petrol station!
Should be Australia wide only at Caltex Starmart sites. Deal lasts four weeks unless "sold out". Discount should be added at checkout, just before payment. If your local Starmart runs out of cards, it should work on printed vouchers too!
Will not work at Caltex Woolworths. Don't ask, they will stare at you like you have two heads.
Yes, a few rural Starmarts are branded Woolworths. If they have Starmart discount tags all over the store, it might work.
Disclosure: Affiliated but not a rep.
Prefer 20% off fuel at Caltex or Caltex start to accept itunes vouchers as payment.