A United Nations representative says proposed New Zealand immigration law changes could violate international refugee conventions.
Under the law change, anyone arriving in a group of 12 or more could be detained under a group warrant.
If they end up with permanent residency those migrants could bring in immediate family, but not extended family as is permitted now.
The office of the United Nations High Commissioner for Refugees is planning to make a submission on the changes.
Regional UNHCR representative Rick Towle says if detention is introduced with a deterrent or a punishment, it could violate international discrimination principles.
Mr Towle says decisions on detention have to be made on the basis of an assessment of the risk a particular person poses.
"The idea of group detention is something that should generally be avoided."
He says the law changes will not deter people from seeking refugee status in New Zealand as those who are desperate to leave their home country will take desperate measures.
Refugee Council of Australia chief executive Paul Power says the New Zealand Government is introducing the worst of Australian policy for no good reason.
Mr Power says it is highly unlikely there will be mass arrivals of asylum seekers by boat in New Zealand due to its geographic position.
"It just amazes me that the New Zealand Government would pass legislation to address a problem which is actually never occurred in the country's history."
The Government says it needs to prepare for the very real possibility a large group will one day arrive in New Zealand.
Prime Minister John Key said on Monday that while the Government has no information there are boat loads of people on their way to New Zealand, it remains a possibility.
Opposition parties and a refugee advocate say the suggested changes are an over-reaction to a non-existent problem.
New Zealand First says the changes are meaningless without dedicated detention centres to house the arrivals.
Labour says the changes are an over-reaction when there have been no mass arrivals in the past, as New Zealand is not as exposed as Australia.
Amnesty International advocacy and government relations manager Amanda Brydon says says the changes discriminate against asylum seekers who want to travel in groups.
Ms Brydon says it is unfair to make it harder for groups and for people who come to New Zealand by boat, and the planned legislation is a sledgehammer approach to a non-existent problem.