It is unconditional, but weaker than the conditional reduction of 10 to 20 per cent the Government had previously tabled in international negotiations.
Greens co-leader Russel Norman described the target as a disgrace.
"As a small country, New Zealand is reliant on major global emitters to reduce their emissions in order to avoid serious consequences of human-caused climate change. We can't expect them to act to save our bacon while we freeload," he said.
Groser said: "I'm very confident on the basis of the advice I'm getting that New Zealand will not only achieve minus 5 per cent, we will end up achieving considerably more than that."
He was still hopeful that the international negotiations would produce an agreement.
"And I am quite confident that any future Government will have to go back and look at the unconditional offer and at some point make a decision as to whether we can lift our contribution beyond this minus 5 unilateral commitment."
He said he agreed with the Government's chief science adviser, Sir Peter Gluckman, that, with New Zealand contributing less than 0.2 per cent of global emissions, any effort it made to reduce them was "more of a geopolitical nature".
"Unless we get a serious international effort, anything we do with the ETS or subsequent to it is a complete waste of time. Disturbingly, over 100 members of this negotiation have not yet said they are going to do anything let alone tell us what it was. And their emissions are not trivial, they are collectively twice the whole of Europe's," Groser said.
"With respect to developing countries which have at least, to their great credit, said they will do things, like China, we are still working through what precisely that means they are going to do. So until we are in a position to make an assessment that there is a serious and concerted international effort, it is entirely appropriate New Zealand sits below its [previous] offer."
Groser said the "strong advice" from officials was that the target could be met without any changes to settings of the domestic emissions trading scheme.
The ETS is widely seen as environmentally ineffectual because of carve-outs from the scheme and the collapse of international carbon prices.
New Zealand's gross emissions are more than 20 per cent above 1990 levels, but it has been able to meet its Kyoto target with a bit to spare because of offsetting credits for the carbon dioxide removed from the atmosphere by forests planted since 1989.
But plantation forests will flip from being a net sink for CO2 to a net source of emissions when the "wall of wood" is harvested in the 2020s.
In the meantime, forestry's net contribution depends in part on what happens to the size of the plantation forest estate.
Groser strongly disputes claims of an upsurge in deforestation now that low carbon prices provide no barrier to landowners switching to another land use.
"There is net afforestation going on," he said. The 26,000ha of plantation forest cut down between 2008 and 2012 was more than offset by 42,700ha of replanting.
"People plant trees primarily for wood. That hasn't changed."
Labour's climate change spokeswoman, Moana Mackey, said yesterday's announcement was disappointing but unsurprising.
"National has gutted the emissions trading scheme, devastated the carbon forestry sector and pulled out of Kyoto."
New Zealand's international reputation had taken a hammering in recent weeks, she said.
"This will do nothing to restore credibility to our clean green image. We are the last developed country to announce a target - a target which was due last year - and we still haven't seen any plan as to how the Government expects to achieve even this very modest reduction."
