"Protecting the health of New Zealanders is our number one priority, and that means we need to reduce the risks associated with large gatherings," Health Minister David Clark said.
And anyone who ignores the new rules could face jail time.
According to the Health Act, anyone found in breach of the new policy could face six months in prison, a $4000 fine or both.
But, speaking to reporters this afternoon, Clark said he expects that people would comply with the new rules.
And he had a message to anyone who has "thumbed their nose" at the current rules.
"You are putting the health of your family, your friends and your country at risk".
He stressed that if anyone was in doubt about an event – even if it is fewer than 100 people – they should cancel.
"For now we must all adopt this new normal".
Meanwhile, Clark said that there was still no evidence of community transmission in New Zealand.
Asked how long the rules would be in place for, he said they would be around for "some time," but added that they would be reviewed.
Anyone who has returned from overseas and needed to be in self-isolation must also stay away from gatherings, Clark said.
"There shouldn't be any grey area around this – if you're meant to be in self-isolation, you don't go to any gathering or event."
The new rules bring New Zealand into line with measures taken by the Australian Government, which banned non-essential indoor gatherings of more than 100 people.
Clark said in workplaces where physical distancing isn't possible, companies should look at other measures which can be used to protect staff, such as working from home.
"We know this has specific implications for the hospitality sector. We will work with the sector over the next 24-36 hours to develop guidance," Clark said.
He said making sure social distancing rules are in place is one of the most important things the Government can do to protect the public.
"These rules will temporarily change the way New Zealanders gather."