The risk of rabies reaching northern Australia is increasing, according to a Sydney University veterinarian.

The virus, which is normally transmitted by dogs, is spreading through Indonesia, and is currently only 300 kilometres from Darwin, in the Tanimbar Islands.

More than 150 people have died of rabies in Bali since the illness arrived there with a rabid dog on a fishing boat four years ago.

If it reaches Papua New Guinea, Dr Helen Scott-Orr says it will be difficult to prevent its spread into Australia.

"It's a matter of when not if," she said.

"The whole of Indonesia is gradually becoming more affluent, travel is increasing to more remote areas as the economy improves, and people take their pets with them.

"Experience in Indonesia is so far once rabies has got into a large island it hasn't been able to be eradicated.

"It could spread not only through the remote islands of eastern Indonesia but also into the very large island of Papua."

The virus kills more than 60,000 people per year worldwide.

Dr Scott-Orr says there are at least 23,000 human movements back and forward between Papua New Guinea, the Torres Strait Islands and Cape York Peninsula every year, making the virus difficult to control.

"It's a numbers game," she said.

"Some people, despite quarantine, will travel with animals because the dog-human bond is very close.

"Another pathway is by yacht travelling to Indonesia or fishing vessel.

"To control rabies, you can vaccinate the dogs, but you would have a very expensive and long-term campaign before rabies could be conquered if it got into Australia."