Eight seasonal workers face deportation after they were found working illegally in Northland.

One Malaysian national and seven Thai nationals, who had originally come to New Zealand as part of the Recognised Seasonal Employers scheme, were arrested following Immigration New Zealand (INZ) investigations into the alleged unlawful employment of migrants.

The workers have been detained and remain in custody in Whangarei pending deportation, INZ says.

Further enquiries are being made with their employers to work out if there have been any breaches of immigration and employment law.

"We hope that this will be a warning to employers that they must make basic checks on seasonal workers before they employ them," INZ spokesman Dean Blakemore said.

"It is also a warning to people who do not comply with their visa conditions - they are vulnerable to exploitation by unscrupulous employers."

Employers employing migrants that are not lawfully entitled to work for them can be prosecuted, and, if they're convicted, they could face a penalty of up to $50,000.