People who knowingly turn a blind eye to child abuse are now liable for a maximum of 10 years in jail under law changes taking effect on Monday.
The Crimes Act has been strengthened in response to the 2006 killing of twin babies Chris and Cru Kahui, which remains unresolved.
Some members of the Kahui family - nicknamed the tight 12 - were accused by police of withholding vital information after the boys' deaths.
Their father, Chris Kahui, was later acquitted of murder and no one else has been charged.
From Monday, law changes make it an offence for anyone over the age of 18 living in the same household or closely connected to the family to fail to act on child abuse they are aware of.
The penalty will also apply to hospital staff who know a child is being mistreated or in danger of being killed.
The law changes also double the maximum penalty for cruelty to a child from five to 10 years in prison.
Justice Minister Judith Collins says the changes send a clear message to adults.
"If you have a child who you know is being mistreated or is in danger of being killed, you must take action.
"But sitting around watching this happen and turning up and saying to police 'I won't talk to you, I won't give evidence' - that's not going to save these people now."
Detective Superintendent Rod Drew says police will put the new law to use.
"Child abuse in New Zealand is a significant problem that we're all working together very hard to try and resolve, so we can reasonably expect that we will use this legislation."
But Lorraine Smith, the barrister who defended Chris Kahui, does not believe the new laws will work, saying they are too simplistic.
Ms Smith says the people at whom the law is directed are often too damaged to report abuse and do not believe police can protect them from the consequences.
