Mr Key confirmed a specialist IED team had been sent to Afghanistan last week after the deaths of Lance Corporals Rory Malone and Pralli Durrer just over two weeks ago.
Chief of Defence Force Lieutenant General Rhys Jones described the insurgents responsible for the deaths of the five New Zealand soldiers as "proficient" and "aggressively trying to hunt us down".
He said the New Zealand Defence Force IED team would work to prevent the further assembly and supply of IEDs.
"It's likely that these IED had already been assembled two weeks ago when that first attack occurred," said General Jones.
Mr Key said New Zealand's Provincial Reconstruction Team (PRT) working in Afghanistan would return to New Zealand in either April or September next year. Cabinet would make a final decision in the next month.
"The Cabinet's preference is more around April - but it's really around the logistics of a number of things that are happening in terms of the way we work with our partners.
"I'm not 100 per cent sure I can publicly tell you the reason why - there is a reason why a later date is probably not possible, even if we wanted to stay longer.
Mr Key said the timing of the withdrawal from Afghanistan was also being negotiated with partnership countries.
"In terms of the argument of 'should we simply cut and run and leave this afternoon' - that is not sensible."
New Zealand has three bases in Afghanistan, as well as 145 troops and equipment which would make a swift exit difficult.
Ten New Zealand troops have now died in Afghanistan.
NZ fatalities in Afghanistan
August 2012: Three New Zealand soldiers serving with the Provincial Reconstruction Team in Bamyan are killed when their vehicle is hit by an improvised explosive device.
August 2012: PRT Lance Corporals Rory Malone and Pralli Durrer are killed in an ambush in northeast Bamyan Province.
April 2012: PRT Corporal Douglas Hughes dies in incident at Romero.
September 2011: SAS Lance Corporal Leon Smith is killed during an operation in Wardak Province.
August 2011: SAS Corporal Doug Grant, 41, is killed during a Taleban attack in Kabul.
February 2011: PRT Private Kirifi Mila is killed in a Humvee accident in Bamyan.
August 2010: PRT Lieutenant Tim O'Donnell is killed in a firefight after an ambush in Bamyan.