The sheep, as yet unnamed, was found by a member of the public near Mulligan's Flat, and rescued by a team of five from the local RSPCA.

The team put out an urgent call for a shearer.

"There could be infections, flystrike, [the coat] could prevent him from going to the bathroom. There could be a really nasty thing under that coat but we won't know that until we get there," RSPCA ACT boss Tammy Ven Dange said.

It had been so long since the overgrown sheep had contact with humans, that just the process of shearing could lead to shock and death, Ms Ven Dange said.

But the sheep will be sedated to give it the best possible chance of survival.

Ms Ven Dange said multiple shearing championship winner and Australian Shearers Hall-of-Famer, Ian Elkins, had answered the call for help.

In response to comments that shearing the sheep amounted to animal cruelty, Ms Ven Dange said that was only true if done incorrectly.

"If it's not sheared, it's actually cruel," she said, adding that merinos had been bred for the specific purpose of growing wool.

But, she said, the sheep had not necessarily been neglected by its owner.

"It happens a lot apparently, when they're trying to herd you can have a bunch of sheep and one gets lost.

"Certainly, the poor guy has been on his own for while."

Early guesses have put the fleece at several years' growth.

A New Zealand sheep which had avoided musterers for six years found fame back in 2004 when he was discovered in a cave with 27 kilograms of unshorn wool.

Dubbed "Shrek" after the fairytale character, New Zealand's most famous sheep was eventually shorn with the proceeds going to charity.

While there are calls to name the Canberra sheep, Ms Ven Dange cautioned the public against becoming attached.

"Just be aware that this fellow might not survive, we just don't know yet, so if we name it now there might be some heartbreak tomorrow."