Police have admitted they were unjustified in arresting a Greymouth man for possession of cannabis.

It follows a report from the Independent Police Conduct Authority into the unlawful detention of three people in May last year.

The authority said the arrest of a man for cannabis possession was unlawful - a point the police have conceded.

The authority also said that a man arrested for assault was held handcuffed in his cell for too long, while a woman also arrested for cannabis possession shouldn't have been kept in her cell for 19 hours.

Police admit they didn't follow correct custodial management practices for either suspect.

They say they have re-emphasised correct practice to all their staff on the West Coast.