Update: The ACT government has announced new restrictions relating to people who have been in Victoria.
ACT Health officials are considering what, if any, extra restrictions to enforce on people travelling from Victoria after the state went back into lockdown.
An announcement on the ACT's response is expected about 2pm Thursday.
The entire state of Victoria will go into a seven-day lockdown from 11.59pm Thursday.
It comes as 12 new locally acquired cases of COVID-19 were detected in the past 24-hours, taking total active cases to 34.
In a statement Acting Premier James Merlino said "this kind of short, sharp action is effective in stopping this thing in its tracks".
The Victorian Health Department has listed 79 exposure sites listed on its website, spread across more than 30 Melbourne suburbs and regional Bendigo, Cohuna and Red Hill.
Health Minister Rachel Stephen-Smith told breakfast radio before the announcement of the lockdown that she had expected more restrictions were coming.
Following a stay-at-home order from Victorian health officials, Ms Stephen-Smith said those who travelled to the ACT from the relevant areas would likely face the same rules here.
IN OTHER NEWS:
Ms Stephen-Smith said that as of 8am no Canberrans had reported being in Melbourne recently at a site identified as at risk.
"For those who have been in greater Melbourne in the last 14 days there are ongoing updates to exposure sites and you also do need to complete an online declaration form," she said.
"If people are listening and they haven't gotten around to filling out the online declaration form, that is incredibly important."
Ms Stephen-Smith said those who registered would receive text message updates from ACT Health as the situation down south developed.
ACT Health is already calling for Canberrans who have recently travelled to Victoria to stay alert and get tested for coronavirus.
From 6pm Tuesday Canberrans who have been to Greater Melbourne in the past 14 days are required to complete an online declaration form.
ACT residents are free to return home from Melbourne but must complete the form within 24 hours of travelling.
Non-ACT residents who have attended close contact exposure sites in Victoria must apply for an exemption to enter the territory.
Several states have already closed their borders to Victoria.
Western Australia will reintroduce a hard border with Victoria, with the state designated a "medium-risk" jurisdiction from 10am Perth time under WA's controlled border regime.
Travel will be blocked for anyone who has been in Victoria since May 16 unless they secure an exemption.
The exemption criteria include senior government officials, active military personnel, federal MPs, transport, freight and logistics workers and individuals approved on compassionate grounds.
"This is a concerning and a fast-developing situation, however the controls we have already put in place have mitigated the risk," Premier Mark McGowan told reporters.
South Australia slammed shut its border for any travellers from Greater Melbourne on Wednesday evening.
SA will only allow essential workers and returning South Australians to cross the border, but they must get three virus tests and isolate for 14 days.
Anybody from Greater Melbourne who travelled to SA since May 20 must also get tested and isolate until they receive a negative result.
"Our thoughts are with everybody in Melbourne. Any outbreak is something we need to take extraordinarily serious," Premier Steven Marshall said.
Tasmanian Premier Peter Gutwein announced all of Victoria will be considered a high-risk location from 2pm on Thursday.
"This means that after 2pm you can't travel to Tasmania if you've spent time in Victoria in the previous 14 days, unless you are approved as an essential traveller," he told reporters.
Tasmanian residents in Victoria can apply for essential traveller status if they want to return home.
However, they must quarantine for 14 days upon arrival at a "suitable premises" or hotel quarantine at their own expense.
IN OTHER NEWS:
On Tuesday, Health Minister Rachel Stephen-Smith discouraged travel to Victoria, saying as the situation evolved it was possible more restrictions would be put on travel.
NSW Premier Gladys Berejiklian says authorities in that state are monitoring Victoria's COVID-19 outbreak "very closely" amid fears it could have crossed the border after people from a NSW sporting club visited one of the exposure sites last weekend.
"I'm worried about the evolving situation and of course we'll watch closely to see what the Victorian government announces this morning," the premier told Nine on Thursday.
"We don't have any intention of closing the borders at this stage, but of course we are very, very concerned with what's going on," Ms Berejiklian said.
"Can I please urge anybody who is intending to get in their car and try and beat the lockdown, please think about your fellow citizens, please be safe, please know your actions can really harm so many others."
Genomic sequencing shows the City of Whittlesea outbreak is linked to the case of a Wollert man, who contracted the virus in quarantine in South Australia earlier this month.
A report released on Wednesday in SA found it likely he caught the virus through aerosol transmission due to the "timing of doors opening and closing" at an adjacent room occupied by an infected guest.
SA on Wednesday became the first state to close its border to residents of Greater Melbourne. NSW and Queensland are urging residents to reconsider travel to the city and parts of regional Victoria.
Victoria's last lockdown - the third since the pandemic began last year - was for five days in February, after the virus escaped hotel quarantine and infected 13 people.