NSW has recorded a worrying rise in Covid-19 cases, with officials confirming 10 new infections in Sydney since yesterday.

NSW Premier Gladys Berejiklian has confirmed one of today's Covid-19 case is not linked to the current cluster in Sydney's eastern suburbs. This case is a student at a school in Waverly.

The NSW Government will extend its current mask restrictions for one more week and the rule will now cover the whole of Sydney.

There are now 21 infections in Sydney's Covid-19 outbreak.

It comes as Queensland confirmed one new locally acquired Covid-19 case on Monday, while Victoria recorded zero new infections.

NSW Premier Gladys Berejiklian. (Photo / News Corp Australia)

NSW Transport Minister Andrew Constance issued a warning to NSW residents this morning, saying they were "clearly in a danger period" of the outbreak.

"I think when you hear the health minister say there is a clear and present danger, he hasn't done that lightly," he told ABC's News Breakfast.

He noted that some Sydney residents seemed to be ignoring the current restrictions around masks.

"Everybody has to play their role as they have over the last year and a half, we have to ringfence the current seeding and make sure we come out of this as quickly as we can," Constance said.

The danger posed by the highly infectious Delta strain of Covid-19 has been made abundantly clear to authorities, with CCTV footage inside Bondi Junction Westfield demonstrating how quickly this strain can spread if nothing is done.

NSW's chief health officer Dr Kerry Chant said the footage showed the virus transmitting between two people during a "momentary crossover" in the shopping centre.

"We know that there's been three people that have been exposed on both June 12 and June 13," she said on Monday.

"In one, we actually have CCTV footage of the encounter and it is basically a crossover of individuals. They are clearly facing each other but it is literally someone moving across from each other for a moment, close, but momentary.

"In two other cases, we haven't been able to, with CCTV footage, look at the exact same crossover point, but we know they were 20 metres [apart], signing in at different venues at the same time or in that area so we suspect they did cross over."

Premier Gladys Berejiklian described the encounter as "scarily fleeting", saying the pair didn't even physically touch each other, with the virus instead being transferred as they came into "the same airspace".

"That's how contagious it is," she said.

The Delta strain, which is believed to be up to 50 per cent more infectious than other variants, first emerged in India and has since spread to dozens of other countries.

It was discovered in the Australian community for the first time earlier this month in Melbourne, when the city was dealing with its own Covid-19 outbreak.

Genomic sequencing has shown the variant on the loose in Sydney matched "perfectly" with a sequence from the US.

Restrictions likely to be extended

As concerns around Sydney's outbreak ramp up, Berejiklian revealed the current restrictions around masks for Greater Sydney, the Blue Mountains, Wollongong and Shellharbour would likely be extended.

The Premier said while yesterday's case numbers weren't a "bad outcome" there was still a level of concern about the way this virus has spread "fleetingly" between some cases.

"Given the situation we are in and given we don't want to see further restrictions imposed more broadly across our city and our state, in all likelihood, the existing settings we have in place will continue beyond the five days," she said.

The list of exposure sites visited by known Covid-19 cases is growing rapidly and now includes suburbs in the west as well as in the Illawarra region.

A number of changes to mask restrictions kicked in at 4pm on Sunday, including the requirement for masks on public transport being extended to include Wollongong and Shellharbour.

Previously this restriction only applied to Greater Sydney and the Blue Mountains, but was extended after a confirmed case spent time in Wollongong.

Berejiklian said these mandates were brought in to help avoid a "superspreader event" that could see cases explode across Sydney and surrounding areas.

Masks are now also mandatory in indoor premises for certain local government areas, including Bayside, Canada Bay, Sydney, Randwick, Inner West, Waverley and Woollahra.

The announcement from Berejiklian follows calls from a top epidemiologist for a snap three-day lockdown in Sydney.

Epidemiologist and World Health Organisation adviser, Professor Mary-Louise McLaws, told ABC radio NSW needs to take this outbreak of the Delta variant "very seriously".

"If the numbers go up any further we should really be thinking about a three-day lockdown," she said yesterday.

McLaws also said masks should be made mandatory in all public areas for Greater Sydney.

Berejiklian has previously refused to rule out bringing in tougher restrictions in order to control the outbreak.

She said the next few days are critical for NSW and any further restrictions "will depend on what cases emerge in the next few days".