Supporters of legalising medicinal cannabis are holding a rally at Parliament today in a bid to make it an election issue.

A group is gathering on Parliament's front lawn this afternoon, just before Parliament sits for the first time this year.

The campaigners want greater access to medicinal cannabis and an end to arrests for cannabis use or possession.

The event's promoters say no cannabis will be smoked at the rally because Parliament's grounds are smoke-free.

"Feel free to bring a vaporiser of edibles to keep this event family friendly," they said on the event's Facebook page.

MPs were asked for their views on legalising cannabis for medicinal and recreational use at Parliament this morning.

Labour leader Andrew Little reiterated that his party supported legalisation for medical purposes and would change the law "very quickly" if it formed a Government after the election in September.

But cannabis would not be legalised for recreational use under a Labour-led Government, he said.

Little said he was concerned about the public health risks of allowing younger people to use cannabis before their brains were fully developed.

Environment Minister Nick Smith said constituents in his Nelson electorate had raised the issue of access to medicinal cannabis with him.

He was "very interested" in seeing the medical advice on the issue before taking a position. He was firmly against legalising cannabis for non-medical use.

National Party MP for Invercargill Sarah Dowie said some of her constituents favoured legalisation for medical treatment so she would "have to look into it a bit more". She was also against legalising recreational cannabis use.

Finance Minister Steven Joyce and National MP Chris Bishop said they supported their party's policy on medical cannabis and would not offer a personal view.

At present, the mouth spray Sativex is the only form of medicinal cannabis available in New Zealand, but is not funded by Pharmac and costs about $1300 a month.

Prescriptions are approved by the Ministry of Health. Other products must be approved by Associate Health Minister Peter Dunne.