About a quarter of MPs say they're undecided about how they will vote on the purchasing age for alcohol.
The Alcohol Reform Bill proposes splitting the age people can buy alcohol, with a purchase age of 18 applying to premises such as bars and restaurants and 20 at supermarkets and liquor stores.
The legislation comes back to the House for its committee stages in June.
MPs will also be voting to either raise the age to 20 for all purchases, or keep it at 18.
Thirty-four MPs contacted by Radio New Zealand say they will vote to keep the age at 18, while 25 indicated support for raising the age to 20.
However, 33 MPs are either undecided or are choosing not to reveal their position. Of this group, 20 are from the National Party.
Supporters of the split vote number about 29 and include Prime Minister John Key and Labour Party leader David Shearer.
The Alcohol Reform Bill also contains new rules restricting the sale of ready to drink alcohol drinks in off-licences, and bans dairies and smaller convenience shops from selling alcohol.