New Zealand's biggest citrus fruit producer, Kerifresh, has been ordered to top up the wages of a fruit picker who was paid a bin rate that equated to less than the legal minimum wage.

Richard McIntosh, a father of two young children, complained to the Labour Department about his pay rates of initially $25, and then $30, for each bin of lemons.

Despite being young, fit and strong, he could pick only two bins a day.

Labour inspector Brendon Hickey calculated that Mr McIntosh's gross pay of $469.58 for 73.5 hours of work in the fortnight ending on September 7 last year worked out at just $6.39 an hour.

He ordered the company to pay Mr McIntosh an extra $191.92 to top up his pay to what was then the legal minimum wage of $9 an hour - since raised to $9.50.

Mr McIntosh received the cheque, minus tax, this month.

The case has forced a major change by orchardists who have complained for years that they cannot get enough seasonal workers at prevailing bin rates.

Kerifresh has just been on a recruitment drive in Samoa and brought back 20 workers who began picking lemons this month.

Kerifresh managing director Alan Thompson said the company now monitored the hours worked by every worker.

"Previously when it was a contract bin rate, if a person wanted to work four hours and go home, they could," he said. "Now we have to monitor every hour they are working. That is a huge cost to monitor both [bins and hours].

"A lot of people like contract work because they can come and go a bit or leave early, and it [the new regulation] is a significant disruption, actually.

"By linking the two measures, we now have to have a number of people making sure every person is at the coalface and is actually working for those hours."

Kerifresh citrus manager Andrew Harty said bin rates were now set so that the average picker could earn $100 a day, or $12.50 an hour - safely above the minimum wage, so that even below-average pickers should earn at least $9.50 an hour.

"Every time there has been a minimum wage increase, the expectation of workers is that they will earn 20 per cent more than that," he said.

"Five years ago when the minimum wage was $7.50 an hour, it [the bin rate] would have been set to earn about $60 a day. The expectation now is $12 an hour. That is the standard throughout the horticultural industry."

Mr Thompson said New Zealand fruitgrowers were struggling to compete against growers in South Africa and South America who paid only a fifth as much.

The legal minimum wage in California was only US$6.75 ($9.22) an hour. 

The 20 Samoans who started this month began on a training rate of $10 an hour for two days and then moved on to bin rates.

Labour Department principal labour inspector Muriel Kelly said the minimum wage law had always extended to piece work and the department had investigated similar complaints in the past in the fruit-picking and clothing industries.

"When investigations show that payments have been below the minimum wage, the department generally seeks voluntary compliance," she said.

"The department can and does take formal action against an employer if the complaint is not resolved informally."