A new mammal species known as the slender root rat has been discovered on an Indonesian island by an Australian scientist and his colleagues.

The rat is about 30 centimetres in length including its tail and weighs about 40 grams.

Scientists believe this rat dines on roots and insects, rather than the insect-only diet of its closest relative, the Sulawesi water rat.

It is unlike anything the researchers have seen before, leading them to believe it is not only a new species, but also a new genus, a broader group on the family tree.

Kevin Rowe, senior curator of mammals from Museum Victoria, was part of the team that found the rat on the remote Mount Gandangdewata.

"According to the local people, who knew the rat, it forages among the roots," Dr Rowe said.

"It seems to have a lot of whiskers ... and they probably help it find food among the moss and the roots that grow in this forest."

The slender rat belongs to a whole new group of rats. ( Supplied: Kevin Rowe )

The scientist have dubbed it Gracilimus radix, meaning slender root rat.

It is the fourth new rat discovered by the team in as many years.

The researchers were led to the rat by local people who farm nearby and are familiar with the forest and its creatures.

"We spent two to three weeks living in a forest camp with local people and what's really remarkable about this area is the close relationship that the local Mamasan people have with the rainforest there and protect the rainforest," Dr Rowe said.

Got a confidential news tip? Email ABC Investigations at investigations@abc.net.au For more sensitive information: Text message using the Signal phone app +61 436 369 072 No system is 100 per cent secure, but the Signal app uses end-to-end encryption and can protect your identity. Please read the terms and conditions.

"Here, the maintainer of the forest is a village elder who goes by the name of Papa Daud but his name is Timothius Sambonanga.

"He was 82 years old when we first arrived in 2011 and he hiked up two days into the forest with us, up the mountain, lived with us and then helped find the animals for us."

Dr Rowe said the island had a wealth of undescribed species hidden in the steep, dense rainforest.

"The trees and the ground itself it covered in a carpet of moss that drapes like a veil from the canopy of the forest and it's very cloudy and misty," he said.

"There's a lot of forest there that's never really been intensively inventoried and so that's been the objective of our project."

Last year the team announced the discovery of the hog-nosed rat.

Another new genus, it is indentified by its hog-like snout, long hind legs, a tiny mouth opening and "curiously" long pubic hair.