A seasonal meat worker who was not re-hired due to suspected cannabis use, despite passing a drug test, has been reinstated and paid more than $20,000 in compensation.

Melissa Mackie worked for four seasons at the South Pacific Meats plant in Invercargill, starting in 2006, but was not re-hired for a fifth season in 2010, an Employment Relations Authority (ERA) finding released today said.

It is standard practice for seasonal meat workers to be laid off at the end of each season and re-employed the next.

Plant manager Malcolm Hampton wrote to Ms Mackie on July 19 to advise her that offers of employment for the new season would be contingent on employees returning a negative drug test.

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Ms Mackie passed the test but Mr Hampton wrote to her on July 30 to inform her she would not be re-hired.

Mr Hampton said the decision not to re-hire her was based on staffing needs at the time and the fact she suffered a workplace shoulder injury at the end of the previous season.

But he also cited her partner Henry Kingi's recent conviction for growing cannabis, which he said raised concerns she may be a cannabis user herself, despite having returned a negative test result.

He was also concerned she had not informed the company that Mr Kingi, who was also an employee at the plant, used cannabis.

Ms Mackie took the case to the ERA, arguing she was unjustifiably dismissed and discriminated against on the grounds of her relationship with Mr Kingi.

The authority did not accept she had been unjustifiably dismissed, saying she was not an employee at the time, but found she was still protected by clauses in the Meat Workers Union's collective agreement.

The clauses set out the criteria for which seasonal workers were to be re-employed by South Pacific Meats, which included length of employment, reliability and the ability to perform the work required.

The authority said while Mr Hampton's conclusion about cannabis use was "a result of mere supposition", he did not breach the collective contract because he was entitled to make his decision based on staffing needs and Ms Mackie's abilities.

But Mr Hampton did fall short by failing to consult Ms Mackie about his concerns over her cannabis use and her shoulder injury.

Authority member David Appleton found the lack of consultation on those issues amounted to "fundamental failings" that fell well short of what a fair and reasonable employer would have done.

He said while Ms Mackie had not been unjustifiably dismissed, she did have a legitimate personal grievance against South Pacific Meats.

Mr Appleton ordered the company reinstate Ms Mackie, pending another drug test.

He also ordered she be paid $9852 in lost wages, $500 for a lost incentive bonus and $10,000 compensation for humiliation, loss of dignity, and injured feelings.