New South Wales Premier Gladys Berejiklian has said contact tracers are working rapidly to link five new cases of community transmission found across the state.

"All that genomic testing is in overdrive at the moment," Berejiklian said.

Sydney was put on high alert yesterday after a 45-year-old man, who transports international aircrew from the airport, tested positive to coronavirus.

A few hours later, a couple, aged in their 60s and 70s from Sydney's Northern Beaches, also tested positive to coronavirus.

An NSW health official alleged the Sydney couple did not isolate while awaiting their results, and their positive diagnosis sparked a flurry of warnings for the region.

The official, not permitted to speak publicly, said the couple "took a long time to track down".

Berejiklian today confirmed two new cases – with the virus spreading to Frenchs Forest in Sydney's north.

The five new infections are NSW's first cases of community transmission since December 3.

Health authorities are pleading with anyone who visited a number of sites on Sydney's Northern Beaches to get tested and isolate immediately.

Dozens of Sydneysiders spent the morning lining up at coronavirus testing clinics. The queues are concentrated outside Mona Vale Hospital, in Sydney's northern beaches region. Health authorities are again urging residents to get a Covid-19 test if they have symptoms.

Other states in Australia are monitoring the situation in NSW closely.

Western Australia reopened its borders to NSW and Victoria last week, dropping the 14-day quarantine requirement that had been in place since earlier this year.

Despite Christmas being just around the corner, Premier Mark McGowan said he wouldn't hesitate to close the border if needed.

"We'll continue to monitor the situation," he said yesterday.

NSW Health Minister Brad Hazzard said other states and territories should let NSW do its job and track the cases – without borders being shut down again.

"We're in the middle of a world-wide pandemic. Australia has done very well. NSW is the gateway to the rest of Australia. Let's face it, it is," Hazzard said.

"We're taking thousands of people in from overseas each week. 45 per cent of those come from places like Queensland, Western Australia, other states and territories. We're doing the work for them. I think they should just allow us to do that work as we've done so well."

The prospects of a transtasman bubble between Australia and New Zealand, revealed in more detail earlier this week, also depend on the Covid-19 situation in both countries not significantly changing.

Prime Minister Jacinda Ardern told TVNZ's Breakfast one of the considerations yet to be resolved was the possibility that if there was a large Covid-19 outbreak in Australia and New Zealand closed its borders, there would still be the ability to bring Kiwis back.

"We have to make decisions on how we potentially quarantine thousands of returning individuals who would then need to come back," Ardern said.

A transtasman bubble is expected in the first quarter of 2021 but Ardern has said an official opening date will only be announced next year after more arrangements between the two countries have been made.