Charlie is an 18-year-old, two-metre, 100-kilogram saltwater crocodile who is rapidly outgrowing his enclosure at the Gold Creek zoo.

The zoo's managing director, Peter Child, said plans for a new enclosure were almost finished, but if Charlie wasn't in there by Christmas he might meet an untimely end.

"If he went [back to Darwin] he would either end up as a handbag or the next steak on your plate," Mr Child said.

"We're not prepared to see his life end that way, not after he's done so much for us and for the people of Canberra."

Crocodiles held in captivity are usually bought from a crocodile farm and raised by the zoo until they're too large. They're then returned and a zoo can get a new crocodile and start the process again.

The problem for Charlie is that having lived in captivity his whole life, he can't be released into the wild.

Male saltwater crocodiles also pose a problem as they can't be kept with other crocodiles because they will fight.

Mr Child said Charlie was the perfect size to be culled for food or leather and he was not prepared to let that happen.

"This was something we always expected, but we didn't expect to grow so attached to him," Mr Child said.

"Our objective now is to build an environmental centre that will house him for the rest of his life."

Charlie has a potential lifespan of more than 70 years.

Mr Child said plans were being finalised and would be subject to government approval.

But should that challenge be met, the zoo will face the issue of raising the $30,000 to $50,000 needed to build the enclosure.

"This will be by far our most ambitious project ever," Mr Child said.

The zoo, which is not-for-profit, is seeking the community's support, either from individuals or, ideally, corporate sponsorship that will get the new enclosure built.

Charlie will require an enormous pond that will need to be enclosed and heated year round. Temperatures below 15 degrees are fatal to a saltwater crocodile.

Mr Child said it would be solar heated and he wanted the enclosure to be "as green as possible".

Mr Child said Charlie would remain a drawcard for the zoo and Canberra more broadly.

Charlie arrived in Canberra in 2015 and when he was smaller he would get out and about, with his mouth bound, so people could pat him and learn about crocodiles.

The zoo's main aim is education and raising awareness of reptiles.

For safety reasons, Charlie can't leave his enclosure anymore, but watching him get fed is still one of the highlights at the zoo.

Mr Child said people often thought Charlie was fake, but he had been known to wink at the odd sceptic.

Hopefully, with community support, Charlie can keep enthralling Canberrans for generations to come.