Police have had to bust up hundreds of parties in the last few days, warning such breaches of the lockdown rules could "waste all the sacrifices" others have made to beat COVID-19.
Mass gatherings remain banned under level 3, as they increase the chance of spreading the disease which has killed 20 Kiwis and more than 240,000 people worldwide.
Since the country moved from level 4 to level 3 last week, police say they've had 1200 reports of mass gatherings. More than half of them - 685 - came between Friday night and Saturday afternoon.
"Under no circumstances should anyone be having a party under the alert level 3 restrictions," Acting Assistant Commissioner Scott Fraser said on Sunday.
"Such behaviour could waste all the sacrifices made by our team of 5 million over the last five weeks. Now is not the time to be complacent and we know from the volume of people reporting these parties and gatherings, that they don't want to lose all the gains made in the collective effort to beat COVID-19."
Gatherings under level 3, outside your home bubble, are banned except for funerals, tangi and wedding ceremonies. Even then, you're only allowed up to 10 people.
Police said they're focusing their efforts on busting up gatherings above that limit.
"Holding a party in the current environment is in breach of the Health Act and where appropriate enforcement action will be taken, this could be against the party hosts and the attendees," said Fraser.
Already 514 people have had police take action against them, and 135 prosecuted. There have also been 342 warnings issued.
The police warning follows Finance Minister Grant Robertson urging Kiwis to not be "idiots".
"I have two clear messages for anyone considering [attending a house party]," he told media from the Beehive Theatrette on Friday.
"First, cancel your plans now. Secondly, be aware that the police will be particularly taking a dim view of this activity as part of their level 3 enforcement activity this weekend... Any uptick in cases as a result of parties will mean that we have to stay in level 3 longer."
The highly contagious virus behind COVID-19 spreads via respiratory droplets, and there is increasing evidence it's airborne too, meaning just breathing the same air as someone who's infected could see you contract the disease.
There have been 1485 confirmed infections in New Zealand.