Photos of a pair of rare giant goannas acting amorously have been snapped near Canberra's city centre.

Historian and amateur photographer Matthew Higgins, who captured the images, said he became involved as a volunteer citizen scientist following a chance sighting of the mighty Rosenberg's Monitor three years ago on Mount Ainslie.

"I identified it as a Rosenberg's Monitor but not everyone I told that story to believed me because I didn't have a camera," Mr Higgins said.

That only hardened Mr Higgins resolve as he began to stake-out the predator that can grow to 1.5 metres in length and live for 30 years.

The goanna hatchlings have colourful facial markings to help tell them apart. ( Supplied: Matthew Higgins )

With a lot of patience and luck, in January Mr Higgins recorded what is believed to be the first footage from Canberra of a breeding pair of the giant goannas, including the female laying eggs.

He christened the female Rosie and her male partner T-Rex.

Mr Higgins said he became fascinated by the way the giant goannas behave.

"They lay their eggs inside termite mounds because a termite mound has a reasonably even warm temperature and high humidity," he said.

"It is a big incubation chamber and it is self-healing because the termites repair any damage."

One of the 25-centimetre hatchlings emerges from the termite mound. ( Supplied: Matthew Higgins )

Mr Higgins said other monitors were among the biggest threats to the Rosenberg's Monitor.

"They maraud around cannibalising one another's eggs," Mr Higgins said.

"So the females stake-out their eggs' stash at the mound, they guard it for two to three weeks."

Mr Higgins said the species' striking facial markings had allowed him to identify 14 hatchlings.

"They are quite beautiful and you can tell individuals from those marks, they are like human fingerprints," he said.

"But there are another couple I can't identify so there could be 16 — Rex and Rosie did a pretty good job."

Public urged to keep dogs on leashes to protect hatchlings

Foxes, dogs and other monitors are the main threats to baby goannas. ( Supplied: ACT Government )

But Mr Higgins said he was warned that a high mortality rate among hatchlings was normal for the species.

ACT Government ecologist Dr Don Fletcher said it was possible none of Rosie and T-Rex's babies would survive until breeding age.

"We see members of the public breaking the rules frequently, we would really appeal to people to people to keep their dogs on the leash under control," Dr Fletcher said.

Dr Fletcher said Mr Higgins' new photos were considered a rarity because feral animals and habitat destruction had decimated the reptile across the south of the mainland.

The monitors are listed as a threatened species in New South Wales, Victoria and South Australia and they are considered rare in the ACT, with just a handful of sightings including within Namadgi National Park in the far south.

Dr Fletcher said he was concerned big goannas like the Rosenberg's Monitor were in decline.

"Sadly we are seeing it on its way out, things aren't looking good for this species," he said.

"Here we are witnessing extinction in our own time."

However a comeback could also be possible, if the predator lizard is approved for introduction to the Mulligan's Flat Woodland Sanctuary - free of feral animals.

"Rosenberg's Monitor is definitely on the list there for consideration and at this stage those decisions have yet to be made," Dr Fletcher said.