Five people have tested positive for use of the drug at the port in the last three months.
Chief executive Kevin Winders said each case resulted in a process through its drug policy involving aspects of rehabilitation and mediation.
However, all of the staff have been dismissed.
"These are team members driving 60-tonne straddles. Their levels have been impaired. Driving a 60-tonne machine, taking a Class A drug, clearly that is serious misconduct."
The positive test results for the drug were unusual and a "concerning and alarming" trend.
All of the company's 325 employees could be randomly tested, but infringements tended to occur at the container area at Port Chalmers, where about 150 people worked.
The company initiated drug tests after incidents, before employment and also randomly tested about one staff member per day.
"We'll be kicking off a campaign and have a provider coming in to raise awareness.
"We'll also be reviewing our drug and alcohol policy.
"The port's testing regime was quite old."
This "clearly could" involve an increase in random testing.
"In some workplaces the testing is 100%; you have to blow before you get in the door.
`We'll be reviewing what some of the other businesses do. We'll see what is a better mix for us."
New Zealand Drug Foundation programmes manager Nathan Brown said it favoured education of managers and supervisors to notice impaired behaviour and intervene, rather than the use of drug tests.
"They are detection tests mostly, which is different to impairment tests like we have with alcohol."
This could result in "moral judgement rather than protecting the workplace".
"There's limited evidence drug tests reduce the number of workplace incidents."
A Customs spokesman said it and police were notified of the issue by the company, but had no information suggesting the drugs came through the port.
A police spokesman said wastewater testing showed an increase in the presence of methamphetamine in the Southern district.
"Methamphetamine can and does have a devastating impact on our communities, especially our youth and vulnerable people.
"We applaud organisations who are taking an active approach to assist their staff in this area."
