The Government is defending its proposal to exempt police, Fire Service and Defence Force bars from liquor licensing laws.

But one of its own MPs is promising to lead the charge to keep the alcohol buying age at 18.

Auckland Central MP Nikki Kaye yesterday pledged to table an amendment to keep the age at 18, even though that is at odds with the Government's preferred option.

"I believe that if someone can be elected to Parliament, get married or join the Army, then they should be able to buy a bottle of wine at a bottle store," she said last night.

"Lifting the purchase age would be ineffective at tackling binge drinking - an issue that affects a much wider cross-section of our society than just 18- and 19-year-olds."

The Government unveiled its alcohol reform plan this week.

It favours a split purchase age of 20 for liquor stores and supermarkets and 18 for bars and restaurants.

But both major parties have said their MPs will be able to vote as they please on any law on the purchase age.

A spokesman for the Prime Minister said John Key had no problem with one of his MPs caucus having a different view on the purchase age.

It is not the first time Ms Kaye has challenged the Government's position - she opposed the proposal to mine on Great Barrier Island, and the Government eventually agreed.

The Government is also under fire for its plan to allow police, Fire Service and Defence Force bars to continue to be exempt from liquor licensing laws.

Casino bars would also be exempted from a national 4am closing time on-licences.

Police Commissioner Howard Broad wanted the exemption for police bars removed, but the Police Association opposed him.

The Fire Service Commission and the United Fire Brigades Association also wanted to keep their canteens, which they said would disappear under the weight of compliance costs unless they were exempted.

Justice Minister Simon Power said removing the exemptions would not tackle alcohol-related harm, particularly regarding young people.

He had been to the bar in the Beehive only twice in 11 years and could not say whether it was a hotspot for harm.

But removing Parliament's exemption was setting a strong example.

Opposition leader Phil Goff said the same rules should apply for all New Zealanders, unless there was a clear reason.

"I haven't seen a case made out for continuing those exemptions yet."

The Law Commission report on alcohol harm "strongly believed" in removing the exemptions.

"Less-controlled access to alcohol should not be used as a reward for commendable public service," the report said.

It noted that by treating the canteens as clubs, compliance costs would be minimised and a manager would not have to be on duty if 20 or fewer people were on the premises.

Reported misbehaviour in police bars includes officers, such as Constable Samuel Withers and Detective Sergeant John Gualter, drink-driving after being at a police bar.

A senior officer allegedly used a neck-hold on a policewoman in a police bar in 2005, and nine Hastings officers were warned after drinking in the police bar when they should have been on patrol in 2004.

Mr Goff also attacked the Government for exempting casinos from any trading hour restrictions, but Mr Key said even the Law Commission did not want that.

He said casinos had to compete with other casinos overseas, and restricting the opening hours would unduly affect the business.

"I don't think people are going to pour out of bars at 4 o'clock in the morning, and go down to the casinos."