Ten people died on New Zealand roads this weekend, including in a pair of triple-fatality crashes.
Police said the weekend's road toll was a "preventable tragedy" and more should be done to make the roads safer including lowering speed limits and removing roadside hazards.
But Mr Key told TV3's Paul Henry programme this morning lowering the speed limit was not the answer.
The main issues for New Zealand drivers were alcohol, speed and fatigue, he said.
Police were doing well targeting dangerous drivers as well as looking at safe speeds but a blanket lowering of speed limits would not be observed by drivers, Mr Key said.
"One person in the weekend is one too many but 10 is terrible.
"Our challenge is in the roads where there's blind corners or people get stuck behind trucks and do silly things or just plain stupid things. The question is where should [police] target?"
Road policing Assistant Commissioner Dave Cliff said it was too early to say what had caused the crashes.
Removing roadside hazards like power poles and trees, lowering speed limits on rural roads and putting wire rope barriers down the centre of roads would bring "enormous value" Mr Cliff said.
"I think we've come to see road deaths now as entirely preventable.
"We understand what the safe system is, it is just the implementation of it, and we are working through that. At the moment, to have so many killed, is all completely unnecessary. I think that is the tragedy of it."
Mr Cliff told TVNZ's Breakfast programme today said the impacts across the community was "huge".
"And that will go on for years."
One way to combat the crashes was to bring the speed limit down on some windy rural roads.
"Take for example the Rimutaka Hill - getting the speed limits down on those portions of roads that are not designed for high speed."
Getting rid of roadside hazards and putting up more median wire barriers would also make a difference to the road toll, Mr Cliff said.
"And there's more we can do in the enforcement domain as well."
If repeat speedsters who were caught by fixed speed cameras had demerit points applied to them, that would make a difference as well, he told Breakfast.
He urged motorists to buy the safest car available if they were planning to replace their vehicles.
He recognised that people did make mistakes that caused crashes.
"The price of making a mistake on the road should not be your death."
Mr Key told Breakfast in the next Budget there would be a lot more money for roading and transport.
Some of that money would go towards improving roads, he said.
AA spokesman Dylan Thomsen said eliminating deaths on New Zealand roads would be a long time coming.
Work was being done by the Ministry of Transport and the New Zealand Transport Association to bring down the numbers, Mr Thomsen told TV3's Paul Henry programme.
"Our rate of deaths per population is about double the lowest in the world.
"The idea of eliminating all deaths and injuries all together may be beyond our lifetime."
But Road safety campaigner Clive Matthew-Wilson told the programme the Government needed to be doing more to make the roads safer.
Simple fixes like fences on the side of roads and down the centre line would make a huge difference, he said."
The anti-speeding campaign this year has been an unmitigated disaster. These accidents are easily and cheaply preventable."
The road toll has hit 126 this year - exactly 20 more than the number of people who had died on New Zealand roads at this time last year.
Ministry of Transport statistics show this weekend saw the highest road toll all year, with the six deaths on Anzac Weekend coming in second.
There have been 110 fatal crashes in 2015 so far with 88 men dying and 38 women dying in the crashes, the numbers show.
February saw the highest number of fatal crashes with 32 on New Zealand roads.
