So.. Melbourne has 2 airports, as I now know.
Realising I had booked Avalon after studying the map I realised it was no good for the car hire and accomodation I had selected and paid for.
No worries, they have a within 24 hours of booking cancellation policy if your trip is over a week away.
Except they don't.
It's only for the US website, Jetstar.com.
Interestingly none of the pages relating to cancellation for the au site work, they have 404 errors page not found.
Apparently not only was it not the right website but my ticket is a starter ticket so is non refundable whatever.
They offered to upgrade the ticket for $820, but then said they would waive this and charge $268 plus $85 per flight ( return ticket) per person to change it to the other Melbourne airport.
It would be cheaper to rebook.
Avoid like the plague.