AT LEAST IT'S DEAD: India Broomhall, 5, and her brother Finlay, 8, with the mummified possum.

What was thought to be a monster rat in Tai Tapu is actually a possum, experts have concluded.

The mummified carcass, which measures 730 millimetres from nose to tail, was discovered last week after property owner Carrie Broomhall saw her dog playing with it on the lawn.

Her veterinarian said it was a rat.

However, several experts, including taxonomists, zoologists and other veterinarians, have confirmed it is actually a dried out, normal-sized possum.

"I'm 99.99 per cent sure it's a possum," said zoologist and retired Canterbury Museum vertebrates curator Geoff Tunnicliffe.

"The lower teeth are almost parallel while rodents' teeth are much more vertical, and the tail is so robust. In rodents, the teeth are really close up and gaps between the teeth are generally never found in rodents.''

Landcare Research spokesman said the hind feet of the animal did not look right for a rat and the tail was also curled, which indicated it was a possum rather than a rodent.

Christchurch veterinarian Eduardo Bernardi wrote: ''Rats have only two upper incisors, as opposed to the four we can see in the picture, and no canines, which are clearly showing in the mummified specimen."

Broomhall said she was really pleased the creature turned out to be a possum.

"It's good to know. You have to be pleased because you don't want a giant rat running around outside.''