Cars were waiting in line at the EPIC testing centre for almost three hours on Friday morning as Canberrans flocked to get tested.

NSW Premier Gladys Berejiklian announced on Friday morning the cluster had swelled to 28 cases, with 25 of those linked to "super spreader events" at the Avalon RSL or the Avalon Bowlo.

Ms Berejiklian said of all the new locally-acquired cases only one was not linked to the cluster and investigations were ongoing for the source of that infection.

ACT Health has directed any Canberrans who had been in the northern beaches since December 11 to self-isolate and get tested for COVID-19, prompting long lines as people turned out in droves.

Residents reported "massive queues" at EPIC on Friday morning, with centres across the ACT experiencing high demand, ACT Health said.

One resident had waited almost three hours at EPIC after she arrived just before 9.30am and was still in line at midday.

ACT Health urged anyone who needed a test to go in the afternoon as workers dealt with the morning rush.

All testing centres are open until 5pm.

The skyrocketing case load has prompted a renewed call from Ms Berejiklian for Sydneysiders to take up mask-wearing.

She said all residents should be wearing a mask in areas where social distancing couldn't be guaranteed including public transport, supermarkets and places of worship.

"It would just be crazy if people are undertaking those activities without wearing a mask. The harder we work together, the better Christmas we will have," Ms Berejiklian said.

The premier thanked northern beaches residents for quickly heeding health advice but said the situation could change rapidly.

She said stricter measures were possible for the northern beaches and possibly greater Sydney.

"I can't rule that out until I know what the next 24 or 48 hours look like. If we get on top of this in the next few days it means all of us can have a much better Christmas," she said.

"At this stage we believe what we have in place is commensurate or matches what the risk is."

NSW Health Minister Brad Hazzard said the cluster was a timely reminder to combat an "avalanche" of complacency which had crept into the community over several months without high numbers of cases.

"Until we have a vaccine we don't have a solution," he said.