@Welkin31 being that your in NSW you will have to find someone who is an AUVIS inspection station with a tradie on site who has a Gas licence. If you can find a shop like that they should either be able to recertify the tank there or direct you to someone who has the appropriate ticket.
If you can find local to you a workshop that does taxi work they should have someone on staff that can help.
As Fourex mentioned above for leak testing a cheap solution is to mix up palmolive dishwashing liquid into a spray bottle with a minimal amount of water, thats what we use at work when we run out of Big Blu, which is a product aimed at leak testing AC equipment and can be bought from any AC parts supplier if you wish to go for that option.
I cannot remember as it has been a hell of a long time since I looked at a LPG Falcon as to where the tank actually sits in those. The petrol tanks are a nightmare to remove as the whole rear suspension cradle has to come out so thats something to consider if its located above the diff, and is definitely not a job I would want to be doing at home, its hard enough on a hoist.
