The ACT will keep travel restrictions for Greater Sydney, Wollongong, the Central Coast and Blue Mountains in place for at least another week.
ACT Health said the public health direction will remain in place until at least January 6. Authorities will reassess this decision on January 5 when they provide the next update.
Residents of the ACT who have been to these places are required to quarantine for 14 days and non-ACT residents who have been in these places should not travel to the territory.
It came as NSW Health announced there were 18 locally acquired infections in the state recorded in the 24 hours to 8pm on Tuesday.
These included cases outside of the northern beaches with cases in Sydney's inner west and Wollongong.
ACT chief health officer Kerryn Coleman said she was concerned transmission could have occurred in Sydney in private residences or functions over the Christmas period.
"There are currently around 160 public locations that COVID-19 cases have visited while infectious in NSW, with more than half of these occurring outside the northern beaches, including in Wollongong. There are nearly 50 public transport routes identified as possible exposure locations in various parts of Sydney," she said.
"Additionally, given the gatherings of families and friends we see at this time of year, there are also other potential opportunities where transmission may have occurred in private residences or functions across Greater Sydney over the Christmas period.
"As a result, we have determined that we need to maintain current requirements for the New Year period, acknowledging the high level of people movement and socialising that normally occurs over this period."
There are currently about 1500 people in quarantine in the ACT, mostly people who have recently been in greater Sydney.
Chief Minister Andrew Barr said the threat of covid would remain high into 2021.
"We just have to accept the fact that most of the new year is going to be just like 2020," he said.
"I know there's a lot of people who want an emotional release at the end of this year and that's fair enough.
"It's been an awful year for everyone, but the stroke of midnight on the 31st does not change the circumstances in which we all find ourselves."
He said reprieve may come towards the end of 2021 with should there be a successful vaccine rollout.
Acting Health Minister Chris Steel said Canberrans needed to celebrate the new year responsibly and practice social distancing.
"We know that many people will be wanting to celebrate the end of 2020 for very good reasons," he said.
"If you're unwell please don't go out and visit a venue, stay home, and of course get tested."