Mr Russell said when there were already heavy workload pressures on permanent staff the stress and inherent dangers were elevated when shift workers with limited knowledge of the standard operating practices came into their working area.
Sara Lunam, corporate services manager at Port of Tauranga, said the port did not directly employ users of the port.
However, every month a Port Users Health and Safety Forum is held to enable workers, including stevedores, marshallers, operators, and their union representatives, and employers to talk about any of their concerns.
Ms Lunam said as far as she was aware this is the first time this concern had been raised publicly. She said she was surprised as there are a number of levels of induction training for any users of the port, and that includes a requirement for all employers to undergo induction on the health and safety rules.
The unions concerns follow the release of results of a global survey which reveal people's stress levels have grown in the past year, with many citing their job as a key pressure point. Mr Russell said union members were already feeling the pressure with limited work availability, and call-in workers were only adding to their stresses.
"Worrying about whether the person next to them knows what they're doing ... is not helping."
Most New Zealand workers who took part in the 16,000-participant survey (by international company Regus) identified customers as being their main source of stress.
Though professional pressures and money worries are a prime cause of stress, one-in-five of those surveyed blamed their "spouse" or "children".
Nearly 40 per cent of New Zealand respondents said their job was the biggest cause of stress in their lives.
More than half of the participants nominated personal finances for the top spot and 54 per cent said dealing with customers caused them the most stress.
Dr Helena Cooper-Thomas, of the University of Auckland School of Psychology, said a worker's performance was often linked to stress and workload.
"If you've got a pressing deadline and you think, 'Oh, my manager is really pushing me' ... and it feels like you're under pressure,' then it will be quite stressful ... and harder to get to," she said.
But, if a manager fostered a culture where workers felt challenged and supported under such deadlines - rather than being pressured and stressed - people were likely to cope much better. "It's about how people interpret work demands ... which can make it less stressful."
Dr Cooper-Thomas said a heavy workload could also lead to increased worker absenteeism and falling productivity.
"Sick-leave research is really variable. We do know that if people are dissatisfied and not involved with their work, there will be more absenteeism," she said.
"An unreasonable workload may also result in people not coming into work."
However, signing up to flexible working conditions would not necessarily provide an instant cure, Dr Cooper-Thomas warned.
Working to hours which suited workers better than the standard nine-to-five could help, but workers needed to ensure strict boundaries were imposed.
"If you're expected to be continually available, than it's not going to be the best thing for someone.
"You have to negotiate strict parameters - both the employee and employer."
Survey findings also show at least one in three New Zealand participants had been more stressed in the last year.
One in five said their partner or spouse was the biggest cause of stress, and 18 per cent blamed their children.
Top 5 causes of stress
Customers (54 per cent)
Personal finances (51 per cent)
Job (39 per cent)
Partner or spouse (21 per cent)
Children (18 per cent)
- Source: Regus
