New police powers over search and surveillance that officials say could tidy up messy, unclear and outdated laws took effect from midnight.
The changes were brought about after the Urewera Raids case, where firearms and other charges against 13 people were dropped after secret police surveillance video was ruled inadmissible because it was not covered by a search warrant.
Assistant Police Commissioner Malcolm Burgess said the new regulations provided certainty about police powers as well as bringing the law in line with new technology.
"If we could actually identify, to a location with sufficient certainty, that particular devices that had been stolen were in that property and we believe they are going to be destroyed or gotten rid of before we could go and get a search warrant - then we could use a warrant-less power then.
"That's a new capability," said Burgess.
But critics of the changes say it could be another dangerous step towards people losing their privacy.
In some cases, law enforcement agents could enter a home without needing a court warrant.
"Those types of powers should always be the subject of the scrutiny of the court before they're carried out," said defence lawyer Jeremy Bioletti.
"What concerns me is how far that is going to go in the end. Are we all going to be permanently surveyed all the time?"
The change has prompted one of the biggest-ever training programmes for police - just under 9000 staff are now across the new law and powers.
The new rules do not affect the secret intelligence service or spy agency the GCSB. Both of these are governed by their own laws.