Legislation reforming liquor laws will be introduced to Parliament on Monday.
Political parties have agreed to support the main package of changes but individual MPs will still have to decide whether the purchasing age should be lowered.
The reforms include tougher rules for parents providing alcohol to young people in their homes and new restrictions on ready-to-drink alcohol.
Parents would need to get approval before supplying alcohol to their children's friends, for example at a party.
Adults who supply alcohol to young people without parental permission could face a fine of up to $2000.
MPs will cast a conscience vote after considering three options for the purchasing age.
A popular option for many is a split purchasing age, with 18-year-olds able to buy alcohol at bars and restaurants and 20-year-olds at off-licences.
Warning from attorney-general
Meanwhile, attorney-general Chris Finlayson is warning that arrest rights police will be given under the liquor laws go too far.
Mr Finlayson says there are several provisions inconsistent with the Bill of Rights, but the new arrest power, for people who breach alcohol bans, is the most serious.
At present, such a breach is a summary offence, carrying a $20,000 penalty. The bill proposes that it be prosecuted as an infringement offence instead.
But Mr Finlayson says it is his understanding that the new law would also allow police to arrest a person for breaching a liquor ban.
He says the fact it would be an infringement offence in itself shows the offence is not serious enough to warrant arrest.
Mr Finlayson says the police already have separate powers to arrest people posing a threat to public safety, or if they're being abusive or disorderly.