Under-18s wanting to drink at a private party, including after-balls, will need their parents' permission under the Government's alcohol reform package, and the host will need to police the level of drunkenness.

Hosts supplying booze without the consent of the parents or failing to supervise the party adequately will incur a criminal conviction and a fine up to $2000.

The proposal is one of several measures of the package, released yesterday following a comprehensive report from the Law Commission.

The Government has identified youth drinking as a major issue.

At present, there are no controls on the supply of alcohol at private parties, even when under-18s are present.

"That has to change," said Justice Minister Simon Power.

The new rules will give parents some control on when their under-18 children can drink in a private setting. A parent can refuse to host a party if they do not have the consent of the teenagers' parents, and the teenagers' parents can refuse to grant consent if they think the party will be raucous.

Mr Power is confident the select committee studying the legislation will clearly define parental consent.

He said examples would include a written note, a text or phone call, implied consent if the host was a close relative, or historical grounds if consent for that host had already been given.

"We've been getting quite a bit of correspondence on this, parents looking for tools to be able to say to their young people, 'Actually, I'm not going to throw a party unless other parents agree that serving liquor in this environment is appropriate'," the minister said.

"The definition of consent will be multi-layered - implied consent or a consent that has come about because of a history of events, for example, [when] it's not uncommon when that person goes to that place to have a glass of wine with dinner."

Police warnings and discretion would prevent a conviction in trivial circumstances even if consent had not been obtained, Mr Power said.

Hosts would also have to be responsible, taking into account the age of those at the party, how well they were supervised, the amount of alcohol, if food was available and the level of drunkenness.

Mr Power said he did not foresee the measures pushing drinking by under-18s into public places. "Under the Summary Offences Act we are going to extend liquor bans to parks, carparks, hospitals and school grounds."

Under-18s can drink in public places only if they are with their parents. The rules will apply to after-balls. If they are held in a bar, the current rules apply: under-18s need to be with their parents and can drink only if their parents buy alcohol for them.

The Government has accepted wholly or in part 126 of the Law Commission's 153 recommendations.

The enabling legislation will be introduced to Parliament in October and should become law after the Rugby World Cup next year.

Police figures show that alcohol was involved in 30 per cent of recorded offences, 34 per cent of family violence callouts and half of all homicides between 1999 and 2008.

The number of licensed premises has increased from 6295 in 1990 to 14,424 in February this year.

"It is clear from these figures that the pendulum has swung too far towards relaxation of alcohol laws," Mr Power said.

The package would empower local authorities to decide where liquor outlets can be established and who could apply for a licence, taking into account the potential impact on the "good order of the locality".

Mr Power said the changes would make licences "harder to get and easier to lose".

The package also recommends a split purchase age for alcohol: 18 for on-licences and 20 for off-licences.

Mr Power said a split age was a good place to start the debate, as it recognised the different environments between on-licences and off-licences.

MPs will likely have a chance to vote on all purchase-age options, as amendments to the legislation are almost certain to be proposed in the House.

Prime Minister John Key, who has two teenage children, said he practised what the proposal was preaching.

"I don't want other parents to supply our children with alcohol unless we have given consent.

"We're quite careful with our children ... and if we have a party at our place, we don't supply alcohol to other youngsters unless consent has been given."

True cost of alcohol
* New Zealanders who die from alcohol-related causes every year: 1000+

* Percentage of police incidents in which alcohol was a factor (1998-2008): 30

* Number of bars and on-licence premises 20 years ago: 2423

* The number today: 7656

* Cost to taxpayers of alcohol-related harm:
$1.2 billion