Some construction companies warn they'll face a shortage of workers if cannabis is legalised in the referendum.
The high-risk industry will continue to test its workers even if it is legal, and worries that will rule out a large group of employees.
Drug testing on construction sites is common practice.
"It's a zero-tolerance policy, we test pre-employment, we test on incidence and we test randomly," said David Howard, Managing Director of Construction Contracts Ltd.
Howard worries it will make it even tougher to find workers.
"We've got a limited pool of people already, potentially going to be more limited," he told Newshub. "[It] might encourage people to use drugs more, become more dependent on drugs."
A survey of almost 200 civil contracting companies found more than 66 percent worry that legalising cannabis will negatively affect them.
"Does this mean we're going to have less people we're able to employ in our business?" asked Peter Silcock, CEO of Civil Contractors New Zealand.
"Does this mean we're going to have a greater drug and alcohol problem in our businesses? I think that's something all employers should be worried about."
But the Drug Foundation wants employers to stop overreacting.
"We've seen in Canada - that's had legislation for over 18 months - that cannabis use in the workplace did not change when it went from illegal to legal," CEO Ross Bell said.
"So let's not think the sky is going to fall in."
If cannabis is legalised, Bell says the workplace urine drug test will need to change. That's because cannabis can last in a person's system for weeks, meaning they could test positive but not be under the influence or impaired.
"There's a whole lot of scientific problems with the tests. Bosses shouldn't require every single employee to pee into a cup to prove they're safe."
The brand new Minister for Workplace Safety, Andrew Little, dismisses the construction industry's concerns.
He says cannabis is already widely available, so legalising it won't make it tougher to find workers.