Aucklanders trying to flee the city's lockdown to head to the bach will be turned back by police, Phil Goff warned.
The Auckland mayor pleaded for people in the city to stay there unless they had a good reason to leave.
"Police will turn you back", Goff said.
Goff, addressing the media this morning, said he will be speaking to the Government on support needed with recovery.
Businesses will be open for click and collect, he said.
Goff urged people not to panic-buy at supermarkets.
Goff says news of lockdown is not unexpected.
"It's something we should expect time and again.
"We need to take extra precaution after the revelation that it is the UK variant."
"We've been here before...we'll beat it again."
Auckland Council made decisions to close public facilities immediately, Goff said.
Goff got a call from director general of health Ashley Bloomfield late yesterday morning as he was on the way to the Big Gay Out, and had conversations with Prime Minister Jacinda Ardern throughout the day
On a Sunday afternoon, this is not the kind of news Aucklanders would like to hear, he said.
Goff said it wasn't yet known if or how the virus had spread from the family who tested positive.
"Until we know we have to act in a precautionary way.
"New cases of Covid-19 in the community was something none of us wanted to happen," Goff said last night.
"However, we always knew there was a risk given what we have seen overseas."
Auckland was raised to alert level 3 as of 11.59pm last night, which will go on until at least 11.59pm Wednesday.
"We know from experience that quick action to impose restrictions is the best way to stamp out the virus," Goff said.
"Aucklanders have more experience than anyone else in the country in dealing with lockdowns. We know what to do to stop Covid-19 from spreading and we will do it again this time."
Goff said this was a good time to remember our border workers who "are doing an amazing job" and "we owe a debt of gratitude to them" for keeping the city safe.
"It's important too for all of us to remember that this not anyone's fault and compassion goes a long way during these times.
"I am not underestimating the impact that another lockdown will have on people's lives and businesses around the city. I will continue to have discussions with government on the support our city needs during and after the lockdown to ensure we can continue our recovery."
He urged people to follow Ministry of Health advice around good hygiene practices, social distancing and using the NZ COVID Tracer app.
"If you have symptoms or you have been at locations at the time they were visited by the cases, make sure you get tested and avoid contact with others."
Deputy Mayor Bill Cashmore said: "As a city and as a country we must take a precautionary approach to maximise our opportunity to stop any spread of this virus, especially the new strains. We have proven that it can be done, and that we can emerge safer and stronger."
All Auckland Council's community facilities will be closed while the city is at alert level 3.
This includes all playgrounds and facilities like park benches and basketball courts, as well as non-essential services such as libraries, pools and leisure centres, service centres, visitor centres, community venues and early childhood education centres.
Essential council services, such as rubbish and recycling, response to noise issues and public transport will continue.
The council is also urging people to use its online services and to visit aucklandcouncil.govt.nz or phone 09 301 010 for further information.