Gladys Berejiklian says plans are underway to ensure frontline health workers and the vulnerable in NSW are the first to get any COVID-19 vaccine.

NSW has clocked up three straight days without recording a single case of locally acquired COVID-19 as Gladys Berejiklian hosed down expectations of a widespread roll out of a potential vaccine.

"The health advice I received is quite optimistic about a potential vaccine in the early part of next year, but it will be limited in the number of people who can receive it," the premier said on Tuesday.

"We're actually drawing up a plan now in NSW to ensure those most vulnerable and of course our health workers and those dealing directly with COVID patients manage to have that vaccine," she said.

Pharmaceutical giant Pfizer has announced its experimental vaccine was more than 90 per cent effective in late-stage clinical trials.

Millions of doses of potential coronavirus vaccinations are already being produced in Australia subject to final clinical trials, with hopes vaccination could begin in March.

Meanwhile, the premier said "the next major milestone for us is opening our Victorian border and we'll be assessing what happens to Victoria in the next couple of weeks".

She also flagged easing restrictions to help the arts and hospitality communities which have been devastated by the pandemic this year.

"We don't want to see any major cultural organisations or institutions collapse ...so we are looking at safe ways of doing that," she said.

She singled out choirs which at the moment are restricted to just five participants, hinting there could be some easing of that regulation.

She warned that Christmas celebrations could remain capped at just 20 visitors to a home, and reiterated that if possible families should book restaurants which can now accommodate parties of 30.

"I'll say to families who are preparing for Christmas and New Year's - please assume that the current health orders are in place - anything beyond this is a bonus," she said.

"For those of us with large extended families, 20 doesn't cut it, but all of us have to adjust because of COVID and that's a sacrifice all of us make to keep each other safe and keep jobs going."

NSW Health said five COVID-19 cases had been diagnosed in returned travellers in hotel quarantine from the 10,058 tests undertaken in the 24 hours to 8pm on Monday night.

The state's Deputy Chief Health Officer Jeremy McAnulty said the Southern Highlands community had responded well to the call for people to get tested.

This follows cases in Moss Vale and southwestern Sydney recently, and detection of virus fragments in sewage sampled from Rouse Hill and Liverpool last week.

Meanwhile, the NSW government stopped short of mandating masks on public transport, instead introducing a new app feature that predicts when trains will be crowded.

Transport Minister Andrew Constance announced the "world first" initiative, saying it would be crucial as people returned to the office.

The Opal Travel app will use real-time data from registered Opal cards and loading patterns on Sydney trains to predict whether a commuter's usual train service is too crowded, and whether physical distancing will be breached.

A push notification will then be sent to the traveller's phone, on an opt-in basis.

Unions and the NSW opposition have been calling for mandatory masks on public transport for months.