Five of those cases were linked to Sydney's northern beaches, taking the so-called Avalon cluster to 144 infections.
Another three locally acquired coronavirus cases were identified in Victoria, on top of three women whose positive tests ended a two-month streak without infections.
It's believed all six cases are tied to outbreaks of the virus in Sydney.
The increase in cases in New South Wales and Victoria has prompted states to re-impose border restrictions.
The government has revised Victoria's status to one of medium risk, effectively barring travel to WA.
Only people granted exemptions will be allowed to enter, while West Australians returning home will be required to quarantine for 14 days.
Any Victorians who arrived in WA on or after December 21 will also need to self- isolate for 14 days from when they first entered the state.
That will apply to about 16,000 people, although some may already have left after Christmas celebrations.
Those still in WA will have to take a coronavirus test on day 11 or at any time if they develop symptoms.
The same rules are already in place in regard to NSW.
Acting Premier and Health Minister Roger Cook said the new measures were based on the latest health advice.
"We're acting with an abundance of caution, and we are acting with the sole intent of keeping West Australians safe."
WA reported two new COVID-19 cases but both were returned travellers in quarantine.
They bring the total number of cases since the start of the pandemic to 861.
SA has also closed its border to NSW but is still monitoring the situation in Victoria.
Police Commissioner Chris Burgess said the government was aware the changed rules would have a significant impact on people currently on flights or about to board flights to WA.
He said they would be greeted by police and directed to isolate for 14 days, or would have the option of returning immediately to Victoria.
"We appreciate that many of those travellers will be surprised that the rules have changed while they're in the air," Mr Burgess said.
"But that is the very nature of this quick changing virus, this pandemic."
In SA, Chief Public Health Officer Nicola Spurrier said revellers should still celebrate, but called on everyone to avoid kissing strangers, or the so-called "pash and dash".
"If you simply can't resist the kiss, make sure you get their name and phone number in case my team needs to follow up with them for contact tracing or on the off chance you want to see them again," she said.
Greater Sydney has been declared a COVID-19 hotspot by the Northern Territory, where authorities say they might crack down further on travellers from elsewhere in NSW and Victoria.
Anyone arriving in the NT from Sydney after midnight Thursday will be subject to a fortnight of mandatory quarantine.
"Further to this, we are watching very closely NSW and particularly Metropolitan Melbourne ... and we won't hesitate to declare further hot spot regions," NT Health Minister Natasha Fyles said on Thursday.
Ms Fyles urged Territorians to reconsider any travel plans they have for the coming weeks.
Victoria's border closure with the whole of NSW comes into effect from 11.59pm 1 January, 2021.
Anyone who arrives back in Victoria from anywhere in NSW after that time will have to go into isolation for 14 days.
Acting Victorian Premier Jacinta Allan said tens of thousands may be impacted by Victoria's decision to close its border to all of NSW.
NSW Premier hits out at partygoers flouting rules
"It does cause disruption to peoples' travel plans, holiday plans and that is going to be difficult for those families, working through that.
"But it is a decision based on the public health advice, based on the case numbers coming out of New South Wales, also [it is also] based on wanting to make sure we lock in the precious gains and sacrifices Victorians have made over 2020."
In Victoria, authorities responded to the latest outbreak with new restrictions for new year celebrations, including a limit of 15 visitors to households and the mandatory use of masks at all indoor venues.
The ACT and Canberra is considered a green zone, meaning residents there will need to apply for a permit to travel to Victoria.
SA Premier Steven Marshall said while the border closure and associated travel restrictions were harsh, they came in response to the latest health advice.
"The reality is they are needed and we've acted promptly to put them in place," he said.
"Every time you put a restriction in place, that's going to have an adverse effect on our economy.
"But a larger spread of coronavirus would be devastating."'
Queensland said it was monitoring the NSW outbreak "very closely".
Situation in NSW remains 'volatile'
NSW Premier Gladys Berejiklian said it was pleasing the number of new cases on Thursday had fallen, but the situation remained "volatile" as more suburbs and venues were added to sites of concern.
The ongoing outbreak also prompted NSW Chief Health Officer Kerry Chant to issue a grim warning that "we are never going back to normal".
"We might get a little bit more normalcy once we have all the population vaccinated, but COVID potentially will change our lives at all times.
"For, literally, years to come, years."
Household gatherings across Greater Sydney - which includes Wollongong, the Central Coast, Nepean and the Blue Mountains - are limited to five people indoors, down from 10, and 30 outdoors, down from 50.
People in Australia must stay at least 1.5 metres away from others.
Please check the relevant guidelines for your state or territory: NSW, Victoria, Queensland, Western Australia, South Australia, Northern Territory, ACT, Tasmania.
If you are experiencing cold or flu symptoms, stay home and arrange a test by calling your doctor or contact the Coronavirus Health Information Hotline on 1800 020 080. News and information is available in 63 languages at sbs.com.au/coronavirus.