Australia's worst-hit state of Victoria appears to be curbing a virus outbreak after a week of tougher restrictions, authorities said Wednesday, with new cases falling in recent days even as fatalities topped records.

Victoria state Premier Daniel Andrews announced 410 new cases and 21 fatalities -- marking Australia's deadliest day so far -- but said the state "certainly" seemed to be flattening the curve.

"If you look at the average over the last seven days, we are seeing (numbers) come down," he told a press conference.

The state has reported around or below 400 new daily cases for four consecutive days, raising hopes that an outbreak centred on Melbourne has been contained after numbers soared above 700 in recent weeks.

Authorities last week introduced an overnight curfew and shut down non-essential businesses in Australia's second-biggest city until at least September 13 -- the toughest rules imposed in the country since the pandemic began.

Andrews cautioned that cases could again rise.

"But with compliance up... and these measures in place, our experts remain firm in the view that this will drive the numbers down," he said.

He said success would depend on people respecting the lockdown rules. "That's what's terribly, terribly challenging."

The virus has ravaged Victoria's aged care homes, with more than 100 residents dying since early August and almost 2,000 active infections linked to the facilities.

Elsewhere in Australia, most regions are enjoying relaxed restrictions as they continue to report few or zero new cases of COVID-19.

Only New South Wales state has been consistently detecting significant numbers of cases since an outbreak emerged in mid-July after an infected Melbourne man visited a packed Sydney pub.

Premier Gladys Berejiklian said the country's most populous region remained "on high alert" due to small but persistent numbers of cases that could not be traced to any known source.

"Whilst numbers have remained stable in New South Wales for the past month, we can't be assured of that moving forward," she told reporters in Sydney.

Berejiklian urged people to wear masks in public to prevent the spread of COVID-19, hinting face coverings could become mandatory if voluntary take-up did not improve.

Australia has recorded just over 22,000 cases of coronavirus to date, with 352 deaths from the virus.

New Zealand could delay election

New Zealand's looming general election could be delayed, Prime Minister Jacinda Ardern warned Wednesday, as the shock re-emergence of the coronavirus sent the country's largest city into lockdown and forced nursing homes nationwide to shut their doors.

Ardern said authorities were scrambling to trace anyone who had been in contact with four Auckland residents whose positive tests on Tuesday ended the country's envied run of 102 days without community transmission.

A three-day stay-at-home order for Auckland -- a city of 1.5 million people -- went into force at lunchtime on Wednesday, ending weeks of near normality, when thousands had flocked to restaurants and filled rugby stadiums.

Panic buying returned to supermarkets, huge queues formed at COVID-19 testing stations and face-masked police manned roadblocks on major roads to enforce the new measures. 

Ardern warned the September 19 election may be have to be delayed if the outbreak could not be contained.

"We're seeking advice from the Electoral Commission, just so that we make sure we have all options open to us," she said. "No decisions yet, as you can imagine, have been made."

New Zealand's parliament had been due to be dissolved Wednesday, to allow the election to go ahead.

In a statement, the legislature said that step "will no longer be held today" but could be done any time before 13 October 2020, potentially pushing the election out by months.

Health officials were also locking down aged care homes across the country because they could act as transmission hotspots.

"I realise how incredibly difficult this will be for those who have loved ones in these facilities, but it's the strongest way we can protect and look after them," Ardern said.

Director-general of health Ashley Bloomfield acknowledged the heartbreak of many Kiwis as they come to terms with the return of a virus many thought had been defeated.

"I know the virus re-emerging in our community has caused alarm and the unknown is scary," he said. "(But) we've been here before, we can get through it if we work together."

- Election 'very difficult' -

New Zealand had been held up by the World Health Organization as an example of how to contain the disease after recording only 22 deaths in a population of five million, and halting community transmission for more than three months.

Ardern described the new cases as "unsettling" but said all efforts were being made to retrace the steps of the Auckland family of four who contracted it from an unknown source.

Her centre-left Labour Party has been riding high in opinion polls, largely on the back of its success containing the virus through a strict seven-week lockdown earlier this year.

With campaigning temporarily halted by the latest virus scare, the conservative National Party said it was open to a delay if circumstances warranted.

"It's going to be very difficult to have an election in mid-September when we are now mid-August. It is very little time," National leader Judith Collins told TV3.

The initial lockdown is only for three days but University of Otago epidemiologist Amanda Kvalsvig said it could last much longer if the source of the infection was not found swiftly.

"The aim is to return to alert level one (New Zealand's lowest) and regain elimination status -- but that won't happen overnight," she said.

"Even after we stop seeing new cases it'll take time and extensive testing to be sure the virus is once more under control."

The outbreak has already eroded some of the everyday freedoms New Zealanders had enjoyed, with Ardern urging Aucklanders to wear masks and restricting gatherings in the city to a maximum of 10 people.

The final match of Super Rugby Aotearoa -- which had been set to take place in front of a sold-out 43,000 crowd at Eden Park on Sunday -- is also in doubt.

The Auckland Blues said its players had been sent home to await advice on whether they can host the weekend's blockbuster match against newly-crowned champions, the Canterbury Crusaders.