Those who do not report child abuse they are aware of could face up to 10 years in prison.
However Lorraine Smith, who represented Mr Kahui, said the law change would not reduce the country's terrible child abuse statistics.
"Some people who do see or suspect abuse, these people do report it," she said. "The very people to whom [the legislation] is directed are often too damaged to have the capacity to report the abuse, because they know there are consequences if they do report it, and they know that the police won't be able to protect them. This is the atmosphere that the abuse of children and vulnerable people happen."
Ms Smith said those within the household where abuse occurs are often victims of the circumstances themselves.
"How is it going to help? How is it going to encourage people who are in a situation where they are living in a dysfunctional household and who themselves are often fractured and damaged and paralysed with fear about the consequences of reporting abuse?"
Ms Smith said nurses and others outside of the family who see abuse usually will not hesitate to report it, regardless of the legislation.
She said the legislation is "an indictment on our society".
"Who would have thought in 2012 Parliament would need to legislate for a parent or a guardian to provide the necessities and protect them from injury? If Parliament needs to force parents and guardians of children and vulnerable others to love them and to give them the necessary things in their lives and protect them from harm, then we are a really sick society.
"I don't think even third world countries need this sort of legal intervention to protect children.
"Incidentally, what I've said does not apply to the Kahui family where the concept of the tight 12, or people not saying what they saw, is simply untrue."
Ms Smith said the issue of child abuse could not be legislated for.
"You've got to find out why these things are happening in a country like New Zealand," she said, adding New Zealand's liberal attitudes to abortion were to blame.
"If you live in a society which refuses to protect the most vulnerable in it and legislates for abortion on demand simply because it is convenient, then you simply can't be surprised when violence permeates every level of society. That's what I think."
However Jigsaw Family Services chief executive Liz Kinley said the law will be effective as people will now have a legal, as well as a moral obligation to help vulnerable children.
"This law will hopefully provide the impetus some families need to take the step to ask for help," Liz Kinley says.
"We know also, though, that it's not always easy to speak up and that to do so may take courage.
"That's why it's important that the public knows where to turn to for help. If they're worried about contacting Child, Youth and Family or the police, they could contact a Jigsaw agency or another local community based child and family service for advice."
Ms Kinley said is not acceptable to plead ignorance or to turn a blind eye when a child is being hurt or neglected.
Detective Superintendent Rod Drew told Radio New Zealand 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."
Another amendment to the law will allow police to pretend to be a person under the age of 16 in order to arrest older people grooming them.
Previously, if a police officer was impersonating an underage child it might not have been an offence for someone else to have made sexual advances on them.
Lobby group Child Matters also welcomed the new law, with academic services manager Amanda Meynell saying it was an important step in strengthening child protection laws.
"However, there is currently a barrier standing in the way of this Act being able to be as effective as it could, and should, be,'' she said.
"This barrier is about the lack of understanding of what child abuse is, the inability for many people to recognise abuse when they see it and know what to do to help. The way to remove this barrier is definitely through training.''
Justice Minister Judith Collins said she was confident the new law would give children the special protection they needed.
"It is no longer acceptable for a person, such as a family member, to claim they were not involved in the abuse of a child when they knew a child was at risk,'' she said.
"The fact that a person lived in a household and knew of abuse makes them involved.''
Along with changes to child protection, the Act also amends the "claim of right'' defence, which was used in 2010 by the three defendants who were charged with burglary and wilful damage at the Government Communications Security Bureau (GCSB) base at Waihopai.
The group was cleared after arguing that they attacked the base to prevent other people suffering at the hands of the station.
Under the new law, defendants will only be able to use the defence in circumstances where they believe they have a personal right to the property concerned.
Knife crime is another area targeted in the law change, with the maximum penalty for possession of an offensive weapon increasing from two to three years' imprisonment.
