More than $1m in lost wages have been recovered for 2,500 workers after a major investigation uncovered widespread problems on fruit and vegetable farms.

The Fair Work ombudsman’s Harvest Trail inquiry has completed 836 investigations in the past five years, involving 444 growers and 194 labour-hire contractors, with the majority picked at random.

More than half of the investigations found breaches of workplace laws including underpayment, falsification of records, deliberate withholding of payslips, non-payments and unauthorised deductions.

But the ombudsman believes the full extent of worker underpayments is significantly higher than the $1,022,698 clawed back for workers who follow fresh fruit, vegetables and wine grapes harvests around Australia.

“Our inquiry highlighted unacceptable practices of underpaying workers in one of Australia’s largest rural industries,” the Fair Work ombudsman Sandra Parker said.

Growers rely heavily on overseas workers who commonly do not have a good understanding of Australian work rights and entitlements, the report found.

“Migrant workers may not seek help because of language and cultural barriers, concerns about visa status, or because they are unaware of their workplace rights,” Parker said.

The vast majority were on working holiday visas, with people in that category particularly vulnerable to exploitation.

As well as underpayments, the FWO found examples of other exploitation including substandard accommodation, transport and extra charges.

During the inquiry, the ombudsman took court action against eight employers for serious alleged breaches of the Fair Work Act, with four actions involving labour-hire contractors.

Six matters have now been finalised, resulting in over $500,000 in penalties, and two remain before court.

The ombudsman handed out 150 formal cautions, 132 infringement notices, 13 compliance notices and seven enforceable undertakings, and commenced eight legal proceedings.

Despite expressing concern about the findings, the vegetable industry is confident actions already undertaken will remove law-breaking operators.

“We categorically condemn the mistreatment of farm workers in any form and this type of behaviour has no place in the horticulture industry,” the Ausveg chair and Victorian vegetable grower Bill Bulmer said.

The Australian Council of Trade Unions secretary, Sally McManus, called for laws to be changed to give workers quick and easy access to justice to recover stolen wages.

“How many more inquiries like this one will it take before the Morrison government admits that wage theft is a national crisis?” McManus asked.