The cluster has now tripled to 17 cases of COVID-19 in just two days, sparking an urgent alert from NSW Health on Thursday evening.

Visitors to seven venues across Avalon and Palm Beach have been told to get tested immediately and isolate for 14 days even if they receive a negative result.

One of those venues, Avalon Bowlo, is where Dan Lecordier and his family ate out at this week and now their Christmas plans have been cancelled.

"I heard about it [the positive cases] last night and the first thing I did was to think about all the places that I'd been to this week," he said.

Mr Lecordier, his wife Claire and their three young children will now be spending Christmas at their Avalon home alone, isolated and behind closed doors.

"We were going to have family over but not now because we've got to isolate for 14 days," he said.

The family were among many hundreds of people who flocked to Mona Vale Hospital's COVID-19 testing clinic on Thursday morning, and while they arrived at 9.30am big crowds meant they only managed to reach the front of the queue at 1.30pm.

Mr Lecordier praised the government's check in system and said it worked to quickly track him down.

"They [NSW Health] called us this morning because we'd scanned in at the venue and they talked us through what we had to do," he said.

Mr Lecordier also praised staff at the COVID-19 testing clinic and said despite the family's long wait to get tested, staff were friendly and helpful handing out bottles of water, snacks and sunscreen.

Three northern beaches suburbs have been included on NSW Health's alert list for venues, and the full list of COVID-19 testing clinics can be found here.