The Invasive Species Council regards deer as Australia's worst emerging vertebrate pest problem, and chief executive Andrew Cox warns the animals could easily spread across the entire mainland.

"The ACT would certainly be in the firing line," he said.

"Wherever you find kangaroos and wallabies now, you'll ultimately find deer. We already know what a problem kangaroos are for motorists."

Asked how long it could take for deer to entrench themselves in ACT suburbia, Mr Cox said it would probably be decades rather than years, but the timeframe would depend on what measures were put in place to stop their spread.

"A serious effort needs to be made," he said.

"Once they're entrenched, it's very hard to get them out. It's much better to try and keep them out of a place before they get there."

Mr Cox said modelling maps showed a significant increase in the range of feral deer between 2009 and 2016.

He said fallow deer were prevalent in the northern half of the territory, covering about 40 to 50 per cent of the ACT, while sambar deer were distributed across the western two-thirds of the ACT, covering more than 60 per cent.

Rusa deer were in a large patch of bush in the Tinderry area, about 50 kilometres to the east of the ACT, he said.

Seven years earlier, the same maps showed feral deer in a very small area in northern Canberra.

An Environment, Planning and Sustainable Development Directorate spokesman said the ACT government's deer sighting register recorded 240 sightings between 1983 and 2013, when it was shut down because sightings had become so common it was no longer useful.

Of the six species of feral deer in Australia, three of them red, sambar and fallow had been reported in the ACT.

Asked whether deer posed a threat to sensitive areas of Namadgi National Park, including Canberra's main source of drinking water in the Cotter catchment, the spokesman said sambar deer were a particular emerging threat.

"Work is underway to develop strategies to manage the future impacts of this species in the sensitive sub-alpine zones," he said. "The ACT government is hosting a two-day expert workshop on sambar deer next week."

The spokesman said the government was also working on strategies to prevent deer moving into the suburbs.

"It has been demonstrated in other areas around the country that feral deer can colonise urban bushland reserves, posing a range of problems from environmental impacts," he said.

"[Potential impacts include] damaging riparian areas, [feeding] on endangered plants, adding additional grazing pressure, impacting on water quality through wallowing, and killing young trees by antler rubbing, through to road hazards and damage to gardens."

The ACT government's remote area cameras have filmed deer rolling in mud on the banks of the Cotter Dam and causing erosion.

Centre for Invasive Species Solutions chief executive Andreas Glanznig said the centre had established Australia's largest research collaboration to tackle feral deer. He praised the ACT government, which was part of the collaboration, for "punching above its weight" as a small jurisdiction.

"To think that deer are going to just sit on the outskirts is a fiction when you look at other areas that have been impacted by deer for a long time," Mr Glanznig said.

Historically, deer control in the ACT focused on opportunistic, ground-based shooting. As sightings have increased, the government has introduced measures including vegetation monitoring and deer control in areas with unacceptably high numbers.

A control program in areas along the Murrumbidgee River has resulted in the removal of 60 fallow deer since 2016, and 430 fallow deer from Googong foreshores since 2014.