A rogue mango seller who was paying foreign workers as little as $2.68 per hour to work on farms in Darwin's rural area has been fined more than $25,000 by the Fair Work Ombudsman (FWO).

Vinai Chaipom was fined $25,412 in the Federal Circuit Court in Darwin on Thursday for underpaying 12 foreign workers over a two-month period in 2015.

The workers, as young as 19, were mostly backpackers from Europe and Asia on 417 working holiday visas, and were hired after they responded to online job advertisements or approached Chaipom for work.

They were keen to earn money to support their travel, and also to become eligible to apply for a second year on their visas by undertaking 88 days' specified paid work in a designated regional area and in certain industries in their first year in Australia, the FWO said.

Chaipom was trading as The Mango Shop, gathering fruit from the Humpty Doo area and selling them at a roadside stall.

Inspectors found he had contracted the workers to perform between four days and just under eight weeks' work of picking, pruning, and weeding on mango farms at Livingstone, Noonamah, and Humpty Doo.

As casual employees, they were entitled to be paid at least $19.45 to $21.61 an hour.

However, four of the workers were not paid at all, and the rest were only paid between $500 and $1,000, or the equivalent of between $2.68 and $4.77 an hour.

Chaipom also failed to issue the backpackers with pay slips, leaving them with no proof they had undertaken the work.

He has been ordered to back-pay the workers in full within 30 days.

Workers are advised to establish their pay rates and slips with employers from the start. ( ABC Rural: Katrina Beavan )

Rogue operators give others a bad name: NT Farmers Association

Fair Work Ombudsman Natalie James said 417 workers should establish from the beginning who their official employer was, and that they would be paid lawful rates and issued with pay slips.

"It can help them avoid [being] exploited and it can help them avoid difficulties applying for a second year on their visas," she said.

CEO of the NT Farmers Association, Greg Owens, said rogue operators gave everyone a bad name.

"We don't like these type of operators being part of the industry," he told the NT Country Hour.

Mr Owens said there was a national push for industry accreditation for labour-hire companies, and that long-term companies were trying to do everything right.

Ms James said the exploitation of vulnerable workers on Australia farms was completely unacceptable conduct.

"This type of treatment … is extremely concerning," she said.

Ms James hoped this would send a message to rogue operators that their actions had serious consequences.