Mark Garnett, with Parks Canada, says the big cannon was cutting-edge technology 120 years ago. (CBC)

Parks Canada has moved two cannons back to their original home on McNabs Island, in Halifax harbour.

The guns were set up to defend the harbour between 1890 and 1910. The larger one weighs 37 tonnes and is as long as a bus.

With a firing range of 12 kilometres, the big gun was cutting-edge technology in its day. It was the first breach-loading cannon in Halifax, which meant crews could load it from behind rather than ram ammunition down the muzzle.

"When this was mounted in 1892 at Fort McNab it was the highest tech piece of artillery on the planet," said Mark Garnett, asset manager for Parks Canada for mainland Nova Scotia.

The cannons spent that last few decades in Point Pleasant Park and York Redoubt, two Halifax parks.

Garnett said the two guns were returned to the island to help visitors understand the history of Halifax's military defences.

"It's about breach-loading artillery, and it's fun to put it back where it belongs," he said. "It's an important part of the story [and] that is the reason Fort McNab was a national historic site."

The two cannons sit on flat-bed trucks while workers clear away ice and snow. In a few days, a crane will lift the guns and put them into place.

The Fort McNab National Historic Site of Canada is expected to reopen to visitors in May.