The Warehouse is being asked why it's demanding proof of citizenship from its workers.

All staff at the Greymouth store, and a number of staff from various other stores, have been asked to provide full birth certificates and passports.

First union national organiser Dennis Maga says the company claims it's not about proof of citizenship, it's about the right to work.

"However our members are quite upset about that because they've been working there for ages - 10 years, 20 years - and yet they have to provide this information."

Mr Maga said the company is legally entitled to check workers are entitled to work here, under a law passed in 2009.

He said The Warehouse has had seven years to request that information, and now suddenly they're asking for it, which he thinks is an unreasonable thing to be doing.

A spokesperson for The Warehouse says they're not looking for proof of citizenship, just proof that staff are entitled to work in the country.

They say they're legally obliged to do that under the New Zealand Immigration Act.

The spokesperson said they're not wishing to cause any offence to employees, they're just trying to ensure all administrative records are up to date.