A champion sheep shearer has taken more than 45 minutes to clip a lost sheep that grew a 47 kg fleece, in what animal welfare experts have declared a world record on Thursday.

The merino sheep, which appeared to be four times the normal size due to its mass of fleece, was spotted by a bushwalker outside of Canberra earlier this week, before the Royal Society for the Prevention of Cruelty to Animals (RSPCA) rescued the animal.

Shearer Ian Elkins relieved the animal of its coat on Thursday, which RSPCA chief executive Tammy Ven Dange said could have threatened its life if it was left much longer.

The 47 kilogram fleece smashed the previous record held by a sheep from New Zealand named Shrek.

Shrek was not shorn for more than six years when it was found, and had a fleece that weighed 27 kg.

Elkins declared he had comfortably "smashed Shrek's record" as he was clipping the sheep.

"I don't think he's been shorn before, and I think he's five or six years old," Elkins told the Australian Broadcasting Corporation (ABC).

Merino sheep are bred to grow wool in Australia, and if they are not properly groomed, could invite diseases such as flystrike and other infections into a herd.

Ven Dange said due to the condition of the sheep and its fleece, it would likely be too matted and dirty to be of any use in production of clothing.

Elkins needed four helpers to complete the 45-minute haircut. The average Australian sheep, at shearing time, has a coat which weighs five kg and takes just three minutes.