Academics, refugee advocates and charities have written an open letter to the government, calling for better protection for asylum seekers who may arrive by boat.
Legislation to amend the law covering mass arrivals - allowing asylum seekers to be held up to 28 days without a warrant - passed its second reading last week.
Auckland University Centre for Asia Pacific Refugee Studies co-director, Professor Jay Marlowe, said children's rights needed to be protected and the government should address concerns that families could be separated by detention.
The letter to Immigration Minister Erica Stanford is signed by more than 40 groups and individuals, who are calling for independent oversight of detention and restrictions on its length.
Stanford said she agreed with them that mass arrival detention should not be in prisons or police cells, and she had redrafted the legislation to make that clear.
The government previously indicated it intended to use the Māngere refugee centre or MIQ-style hotels in the event of a mass arrival.
When the bill was introduced by the Labour government last year, then immigration minister Michael Wood said the four-week maximum detention was aimed at ensuring large groups of migrants or asylum seekers would have time to access full legal advice, in a country with insufficient numbers of specialist lawyers.
Prof Marlowe said the bill had been rushed through Parliament this year, and there were risks as a result.
"To keep families together is certainly important [as an issue] to address," he said. "Also independent oversight, so that there would be this body that could look into that detention, and within that maybe the Ombudsman or even the ICRC [International Committee of the Red Cross] could be involved in that."
The group also wants to see 'calibration' in the bill to differentiate between sizes of asylum seeker groups, allowing a sliding length of maximum detention times for them.
"For example, if only 30-50 people arrived, there would be guidelines on how long they could be detained through the Warrant of Commitment process, which would differ from if there were 300 or more people arriving," Prof Marlowe said.
"We also want to emphasise that there actually are alternatives to detention, that people can live in the community, with or without conditions and limited restrictions. We wanted to make that clear, so that that detention did not become a fait accompli around a supposed mass arrival if it were to actually eventuate."
Although New Zealand has never had asylum seekers arrive by sea, other asylum claimants are mostly in the community while they await a refugee decision.
A small number have been held in prison, but the practice was criticised by a government-commissioned report in 2022, after revelations which included the prison rape of a man who was later recognised as a refugee, and coercion by criminals into taking part in fight clubs.
A spokesperson for the minister said protections for children and family unity existed in the operations manual, Immigration New Zealand policy and other legislation and international agreements.