As United Future shuts down, its leader fears any chance of legalising cannabis will stall.
Damian Light said for him personally, United Future's biggest achievement was the drug reform measures that were implemented during its time in government.
"It's unfinished though, that's the problem. It needs to be carried on, and that's one of our concerns, is who's going to carry it on," the United Future leader told The AM Show.
"Labour doesn't seem very interested, they've not given it to the Greens and we know the Greens are the ones with the drug reform experience and understanding."
Deaths from so-called synthetic cannabis showed why the matter needed to be pursued.
"We need to be actually taking this forward, we need to be having further conversations about it. We need to be looking at legalising and regulating real cannabis, recreational cannabis," Light said.
"It absolutely has to be done. It's proven to be low risk. We need to move it into the regulated space."
United Future's lone MP Peter Dunne decided to quit politics in August, just a month out from the general election. Polling in his Ohariu electorate had convinced him voters were in a mood for change.
United Future was unable to win any electorate seats in the September election and picked up fewer than 1800 party votes.
"We had a lot of people saying to us this time they didn't like the party because we went with National even though we'd previously gone with Labour, and would have gone with Labour again if Labour had got in, which they did," Light said.
"We've always said we will work with the government of the day, but people have got very short memories."
United Future said its 15 years in government, working with Labour and then National, was the longest continuous role in New Zealand government for 100 years.
Light said the decision to close down the party was bittersweet.
"We've been looking at the party, thinking about our future, reflecting on how things have gone over the last 15 years.
"We're really proud of where we've come from, and what we've done, what we've achieved but, at the same time, we've decided that it's time to call it a day, and so it's time to retire the party," he said.
Personally, he had always had an interest in politics and wanted to stay around, but hadn't been looking at his future, Light said.
United Future members might look at forming a rebranded party.
Light was asked if he was planning to set up a new party that would look for an electoral accommodation with National to get it over the line at the next election.
"My view is we don't look to support a particular party. We might look at setting up another party, that's an option. There are other parties out there as well," Light said.
He said he always felt uncomfortable about electoral accommodations.