Australian animal welfare officers Wednesday put out an urgent appeal for shearers after finding a huge sheep with wool so overgrown its life was in danger, with a national champion set to take on the challenge.

The very woolly merino sheep was spotted wandering on its own near Mulligan Flats, a grassy woodland just outside the national capital Canberra, by bushwalkers who alerted local RSPCA officers.

The officers headed out to the area, but had to return on Wednesday morning with reinforcements after noting the creature's size.

"It's definitely one of the biggest sheep we've ever seen," said Tammy Ven Dange, head of the RSPCA in the Australian Capital Territory, adding that the as-yet-unnamed animal was "four to five times its normal size".

The extent of the wool growth and the sheep's nervousness around human beings after what was likely years of solitude also meant rescuers were not yet able to confirm its gender, age and weight, although they thought it was male.

"He's pretty stressed out at the moment. We're trying to keep him calm and hopefully tomorrow (Thursday) we're going to sedate him and shear him," Ven Dange told AFP.

"Hopefully he doesn't go into shock during that process."

Merino sheep are bred for their sought-after wool and can experience health issues, or even die, if not sheared regularly, Ven Dange said, explaining why she tweeted her call for help.

Ian Elkins, a national champion inducted into the Australian Shearers' Hall of Fame, responded to the appeal and said removing the fleece "could be one of my biggest challenges yet", with the process expected to take between one and two hours.

"They sent through a photo and I've never seen such wool on a sheep before," Elkins told AFP, noting that on average, a sheep would grow seven centimetres (2.75 inches) of wool each year.

The animal's voluminous fleece could rival New Zealand's most famous sheep, Shrek, which was found in 2004 following six years on the loose after going missing from its herd in 1998.

Shrek, also a merino, was found in a mountain cave and was shorn of nearly 27-kilograms (59 pounds) of fleece in an operation broadcast live around the world. It was put down in 2011 aged 16 due to failing health.