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A landmark employment case sends a strong message to employers to be sure they are right if they discriminate on gender, an employment law expert says.

Napier employment law advocate Gary Tayler said a Wellington High Court ruling in favour of Motueka woman, Caitlin Lewis showed employers must have a "darned good reason" for hiring people based on gender.

Ms Lewis complained that because she was a woman, she was given a lower paid job than men at the Talley's fish processing plant in Motueka.

She was given a job as a fish trimmer because she was a woman but her partner got a job as a filleter, which paid more, because he was a man.

A High Court ruling in Wellington found yesterday that fishing company Talley's discriminated against Ms Lewis.

Mr Tayler said there were grounds for gender discrimination but the ruling showed that employers must be very sure they were justified if they hired people because of their gender.

Justice Simon France said Talley's argued filleters received a higher pay rate than trimmers because their work was more demanding and important.

Talley's had also confirmed, with few exceptions men at the Motueka factory were filleters and women were trimmers.

He said Ms Lewis was paid less because she was made a trimmer and the reason she was made a trimmer was she was a woman.

"Talley's did not directly pay her less because she was a woman, but discrimination need not be deliberate," the judge said.

Mr Tayler said if an employee was a woman she could not get a job as a filleter.

"You can't be a filleter because you are a woman, that is really the guts of it. If you weren't a woman you could be a filleter and get the higher rate."

He said employers had to be sure they were following the legislation and adopting a valid reason for discriminating on a gender basis. That reason had to be an exemption allowed in law.

"The question really is not 'are you discriminating', but 'why are you discriminating' and it is the answer to that question which tells you whether you are in breach (of the law).

"If they don't have a good answer they will get caught," Mr Tayler said.

"They will have to be a darned good reason, one that will stand up in court," he said.