The withdrawal of New Zealand troops from Afghanistan hinges on Japan's plans to redevelop the Bamyan airport, Prime Minister John Key says.
PHOTOS: NZ soldiers killed in Afghanistan
He says the only windows for a 2013 withdrawal of about 145 New Zealand soldiers are in April and September - with April the government's preferred option.
On Monday, he told media the withdrawal relies on negotiations with an unnamed coalition partner.
Mr Key has now revealed that partner is Japan, which plans to shut down Bamyan airport in May for a redevelopment, as part of its aid programme.
"Once they start that work, it renders the airport unusable by our people and therefore we can't use it to exit from Kiwi base, which is in Bamyan," Mr Key said.
He said the Ministry of Foreign Affairs and Trade has been in discussions with the Japanese government about the starting date of that work for around a month.
Japan named May as its preferred start date about two weeks ago.
If New Zealand forces don't leave before the airport redevelopment begins, they will have to travel about 180km by road to Kabul to fly out.
"I think that road is too risky. It would mean that we would have to have people in unprotected vehicles, and it's just not an acceptable way to get 145 people out of Afghanistan."
Mr Key said New Zealand also relies on backup air and military capability from the US Armed Forces, which is another factor in the withdrawal date.
He said it is also likely New Zealand troops will be pulled out of the Do Abe and Romero bases in Bamyan's northeast before winter hits.
"They, because of the snow and the conditions up there, almost certainly would have to come out (before winter)."
Five New Zealand members of New Zealand's Provincial Reconstruction Team have been killed in the northeast area this month, doubling the number of New Zealanders killed in action in Afghanistan.
In the latest incident on Sunday, Lance Corporal Jacinda Baker, Corporal Luke Tamatea and Private Richard Harris were killed instantly when a bomb exploded under their Humvee.
Mr Key said the bodies of the three soldiers are now expected to arrive home on Thursday afternoon.