A total of four arrests have been made over synthetic drugs in west Auckland - in two separate sting operations.

Three men aged between 22 and 33 have been arrested for supplying synthetic drugs.

A 58-year-old man is facing six charges related to manufacturing and importing psychoactive substances.

Detective Inspector John Sutton said that $1.5 million worth of synthetics have been linked to the 58-year-old.

However, he said there's still more work to be done.

"It's an ongoing investigation, and we stress that we'd love the public to get in touch with us and give us any information they can that can help us."