Bill English says the fact that drugs are already illegal justifies them staying that way.
It comes as a two-day symposium on the issue of decriminalisation finishes up at Parliament.
The Prime Minister told Leighton Smith the topic's been debated in New Zealand for fifty years.
"The fact that these drugs are still regarded as illegal tells you that as a society we have considered the harm to be great enough to make them illegal."
WATCH THE FULL INTERVIEW WITH LEIGHTON SMITH AND BILL ENGLISH ABOVE