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."