Generally speaking, the longer you quarantine a new pet from the rest of your collection the better. Live plants should also be quarantined.
I sequester plants for 30 days.
For animals, it depends on the source for me. Animals from a pet shop or less than reputable sources can get up to 90 days.
Keep in mind that unless you practice good sanitation, quarantine is a futile effort. It is best to keep a separate container (plastic tubs to the rescue!), air pump/stones, siphon, net and the like just for quarantine purposes. These items should be kept away from your regular gear and should never be used interchangeably to prevent possible cross contamination.
Since you live in Australia, it would be good to take your axolotl in to see the vet during the quarantine period. (Seems they have vets down under that actually know what an axolotl is!
) This period is the perfect time to prevent/treat serious health issues without putting your other axolotls at risk.
It may seem paranoid to go through all this, but I speak from experience. I once managed to both infect my tanks with snails after adding new plants and kill off close to two dozen juvenile axolotls when I introduced a new axolotl that carried an infection to the tank. I lost all of those animals in less than a week.