Auckland's mayor has called for calm and urged Aucklanders not to panic-buy as retailers warn that the return to lockdown could be "catastrophic" for struggling businesses.

Auckland will be plunged back into an alert level 3 lockdown at midday today following the news that four new cases of Covid-19 have been detected in South Auckland.

The restrictions will last until midnight on Friday. The rest of the country will move to alert level 2 as a precaution.

Auckland Chamber of Commerce CEO Michael Barnett told Mike Hosking this could be destructive and confusing for businesses, which are low on reserves and light on confidence.

He says if the Government has a plan, they should give it to businesses, not just leak it out piecemeal.

Retail NZ CEO Greg Harford said the lockdown would be devastating for a struggling sector and another nail in the coffin for many businesses.

"The retail sector has been struggling to recover from the effects of the earlier lockdowns, and spending since March is still down nearly 10 per cent," Harford said.

"A further lockdown has the potential to be catastrophic for struggling businesses, and Retail NZ is hoping that the new lockdown will be lifted quickly once the full situation is known and managed."

He also urged shoppers to be calm and avoid panic-buying.

"Essential services such as supermarkets will remain open, even at alert level 3. Retail NZ is calling for everyone to stay calm and shop normally."

He also asked whether additional financial support needed to be made available for affected businesses.

Goff also called for calm and patience as another lockdown looms over the city.

"I understand that people are probably feeling a little bit scared, a little angry and a little confused right now. None of us wanted to go back into a lockdown, but we always knew this was a very real possibility," he said.

"I am urging Aucklanders to come together like we did last time to stamp out community transmission. Please remain calm, please do not panic-buy and please follow the lockdown rules.

"Please follow Ministry of Health advice around good hygiene practices and social distancing. If you have symptoms, please make sure you get tested and avoid contact with others.

"We beat community transmission once and we can do again, but that requires all of us to work as a team.

"Like last time, more details will be forthcoming over the next little while, so please keep updated through news channel and official government websites," said Goff.

Manurewa-Papakura councillor Daniel Newman said his constituents in South Auckland are anxious about the threat posed by Covid-19, the disruption to their lives and the economic consequences of a return to a lockdown.

"Testing along with other measures such as social distancing will be fundamental to stamping out the scourge of community transmission," Newman said.

He said the four new Covid cases in South Auckland from unknown sources illustrates the need for a significant increase in testing and an expansion of the criteria to include both symptomatic and asymptomatic people.

"We're heading back to level 3 and my community is at the forefront of these restrictions. But we have been at risk of a possible outbreak of Covid-19 since we came out of the lockdown.

"Testing must extend beyond those who are symptomatic and people who are working at the borders. Covid-19 is tricky and applying narrow criteria for testing will not help people who are asymptomatic," Newman said.

Harford said the return to lockdown would dent public confidence and could mean customers were less likely to get out and about.

"I guess the key message for the rest of New Zealand is that there is still no evidence of Covid-19 being transmitted in other parts of the country."

The Government would need to remain very transparent about what was going exactly with the new level 3 lockdown, he said.

"I think the more information businesses have about how long this is likely to go on for the better", he said.

"Most importantly, there may be a need for additional government financial support to be made available to businesses."

Elsewhere, Restaurant Association chief executive Marisa Bidois said that without a doubt the announcement tonight would shock the entire industry.

There will be financial impacts on businesses in the restaurant industry, she told the Herald.

"Our numbers have been showing some really positive signs of improvement - the school holiday trading period has been really positive, there was definitely more optimism in our sector that there has for the last three months," Bidois said.

"Our immediate reaction is worry, shock, concern."

The Restaurant Association was working to put as much information on its website about the new lockdown.