Prime Minister John Key says he received an intelligence report just before Christmas that a boat from Indonesia with hundreds of asylum seekers was planning to head to New Zealand.
He also revealed that the Australians have considered whether they would help shepherd a boat across the Tasman to New Zealand if the people on board were adamant about getting to New Zealand.
Mr Key was speaking at his post cabinet press conference about the deal he has made with Australian Prime Minister Julia Gillard to take 150 approved refugees a year from Australia - people who have been processed in Nauru or Manus Island in Papua New Guinea.
He said New Zealand had to pull its weight and that the possibility of a mass arrival in New Zealand was real. He was dealing an instance just before Christmas.
"I can't publicly release a lot of information, even under the [Official Information Act], for obvious reasons but what I can tell you is that I get more than enough correspondence from my intelligence agencies fed from the Australian intelligence agencies to tell you that this situation and the likelihood of a boat wanting to come to New Zealand is very real and very alive and I was dealing with it just before my Christmas holidays."
It involved hundreds of people but the voyage was stopped.
"Either it fell over or they had enough information to ensure it didn't take place. All I'm telling you is this stuff is real. I deal with it all the time and at some point someone's going to get a boat that 's going to turn up."
Mr Key said the 150 would have to have been through the Australian ''no advantage' system, meaning they would have been in detention for the same estimated time they would have to have waited in a land camp, had they not arrived by boat.
Mr Key reiterated the point that if there was a mass arrival of asylum seekers in New Zealanders, that Australia would be receptive to processing them at one of its offshore centres.
But he was in no hurry to change the law to effect that. That would probably be addressed only after such an arrival.
He said the refugee resettlement centre at Mangere was in the midst of an upgrade, but it was not being expanded in anticipation of a mass arrival.
Legislation setting out the process for dealing with mass arrivals - deemed 10 people or more - including the right to detain them in military facilities is languishing on Parliament's agenda. But Mr Key hopes that will be passed this term.
Mr Key said the deal was at no cost to New Zealand because the 150 came out of the 750 quota New Zealand has promised the United Nations High Commission on Refugees it will take annually.
He said New Zealand had been keeping the UNHCR appraised of its thinking on the matter well before the announcement at the weekend.
"In the end they understand we have our sovereign right to choose where we take refugees from."