That truth was that Canberra's borders with NSW are so porous and many that to effectively police them all and keep potentially infected interlopers out was, as one senior ACT officer described it, "a nightmare".

Agitation within the ACT government began when the Avalon outbreak, as it is now being described, quickly spread from mid-December into the greater Sydney area, prompting governments to also classify the Blue Mountains and Wollongong as hotpsots.

Contact tracing in NSW was ramped up as multiple virus hotspots appeared like weeds after rain.

Just a few hours' drive down the Hume Highway, speculation then began within ACT Health of what action could be taken to protect the citizenry of Canberra.

This latest outbreak was different in nature to earlier in the year when the fast emergence of the pandemic threat forced NSW and the ACT into a commonsense restriction lockstep.

But this Sydney outbreak was more compartmentalised and left the ACT highly exposed as the word quickly spread, fuelled by social media, that conditions were going to change very quickly.

Then came news that 14-day quarantine restrictions, stretching into Christmas, would kick in at midnight on Sunday December 20. Within hours, people began to activate their travel plans early.

Thousands hit the road, including Canberrans eager to get home and Sydneysiders just eager to get out. ACT residents planning to bring family to them for Christmas cranked their Plan Bs into action, with one Canberra business manager describing how within an hour he developed a plan to drive direct to Port Stephens, collect his elderly mother, and head straight back.

The Hume and Federal highways were thick with Sunday traffic heading southward, as were many of the other main arterials out of Sydney. The great exodus had begun.

From midnight on Sunday, people entering the ACT after being in the greater Sydney area - encompassing the Blue Mountains and south to Wollongong - were required to self-report and go into quarantine.

Confusing matters further was the mistaken declaration by the ACT government - soon after quickly withdrawn - that the wider Illawarra and Shoalhaven area was to be included in the affected zones.

After last summer when the Kings Highway closure kept thousands of Canberrans away from their shacks and South Coast holiday plans, this sent a huge ripple of concern through the community.

ACT chief heath officer Dr Kerryn Coleman flagged the idea of a border police "presence" early on Monday morning on ABC breakfast radio. This early and unexpected call from the lead agency had the police scrambling.

But as a small and closeknit workforce, ACT police have always been good scramblers.

First there was the suggestion from police of a border checkpoint being set up southbound on the Federal Highway as a "training exercise".

However, the public optics of a "training exercise" didn't match the seriousness of the situation and by Monday afternoon, with the services of 31 eager new probationary constables straight of the AFP Police College at Barton to call on, ACT police announced that a Federal Highway checkpoint would go ahead with "random checks" planned on travellers.

The language police used was careful; this was never intended to be a lockdown or a closure such as had existed between NSW and Queensland.

The southbound Federal Highway checkpoint was set up, complete with traffic funnelling and message boards, described by police as "random interceptions to speak with drivers about the new public health directions and any requirements to quarantine in the ACT".

Deputy Chief Police Officer Michael Chew said there was planning around border strategies back in March and April but admitted resourcing those strategies was identified as an issue then, and would be again.

"We're not at a stage where we will be checking every vehicle or every person that comes back into Canberra through the roads," he said.

"We're looking at some presence on the Federal Highway to capture traffic coming down the highway that would predominantly be from Sydney."

Police reported that more than 2700 drivers were spoken to during the first day shift of the Federal Highway interceptions.

However, inbound highway travellers from Goulburn and further north reported the checkpoint was gone the next day with only the LED message boards still flashing their warnings of the need for people from Sydney to quarantine. It was hardly surprising; sustaining the effort was always going to be difficult.

Dr Coleman and the police admitted that their strategy was one in which they relied on people to "do the right thing". It's like a social contract; always a tenuous and hopeful concept if other parties involved are hell-bent on doing exactly what they want, with little chance of being caught.

From the Monaro Highway to the south, the Barton to the west, Kings Highway to the east and the Federal to the north, plus a myriad of other roads from the Uriarra to Oaks Estate and all the way down to Googong, the ACT and NSW are entangled with ribbons of bitumen.

Getting a cop on every ACT border access road was never going to happen and the draw-down needed from all other key community policing functions makes it nearly unworkable.

But consider this: unlike every other jurisdiction in Australia, the ACT effectively has two police forces. The ACT's officers are all Australian Federal Police members, their services paid for by the ACT government. Yet they all serve under the direction of Commissioner Reece Kershaw.

Only on very rare occasions - the 2008 Beijing Olympic Torch Relay in Canberra being the most recent, calling in a flood of police resources to prevent a repetition of the massive anti-China protests elsewhere the torch travelled - have sworn members working at AFP headquarters in Barton been directed to don their blue uniforms and join their ACT colleagues on the front line. But it worked then and could so again - at a pinch.