Next year's cannabis referendum will fail to see the drug legalised as growing numbers of Kiwis turn against legalising recreational cannabis, a new poll suggests.

New Zealanders will vote on whether to legalise the drug at the 2020 election. But Horizon Research's polling shows support has fallen from 60 percent in November, to 52 percent in April, to just 39 percent this month.

And the number of adults against legalising the personal use of cannabis has nearly doubled since November - increasing from 24 percent to 47 percent.

Paul Manning, executive director of Helius Therapeutics, says he's not surprised New Zealand is turning against legalisation as key messages from cannabis proponents are not getting through.

"On the current trajectory, the referendum is heading for certain defeat," he said on Saturday.

"A vacuum of information in recent months has been filled by fear. This is the first set of comparable independent surveys, conducted by the same research company, and the downward trend is staggering. Those pushing for liberalisation will be particularly alarmed to see the substantial drop-off in support, particularly from Green Party voters and younger Kiwis."