If the Darwin wreckers don't have one, try others interstate. Otherwise you can get a competent boilermaker or panel shop to cut any rust out. There are also products which seal the inside of the tank but it really depends on what the problem is with the old tank.
"If anyone disagrees with anything I say, I am quite prepared not only to retract it, but also to deny under oath that I ever said it." --T. Lehrer
"The secret of success is sincerity. Once you can fake that you've got it made." -- Jean Giraudoux