"For the first time, New Zealand is proposing a law that would lock up boat-arriving asylum seekers. The irony - in modern history, New Zealand has never had a boat arrival of asylum seekers," the group's Facebook page reads.
"Mandatory detention has had disastrous results for Australia - suicides, depression, detainees burning their camps in despair. Not so in this country.
"New Zealand has always allowed the very small numbers who do make it here by plane to wait in the community until their case is decided, with few exceptions. Only about 125 asylum seekers get to stay in New Zealand, of the roughly 300 that apply each year. We have always honoured the UN Conventions that promise we won't treat those running for their lives like criminals - until now."
Immigration Minister Nathan Guy has defended the bill, telling Parliament in May that although New Zealand has never had a "mass arrival" of asylum seekers, the law change will enable the country to act if it ever occurred.