Key Points:

Police have found a methamphetamine lab at the old Matangi Dairy factory.

Waikato police found chemicals used in the manufacture of methamphetamine, or P, at the factory east of Hamilton.

Two men and a woman have been arrested and charged with manufacturing the drug and having chemicals to make P.

Firefighters had to be called to decontaminate the trio and police.

Waikato police spokesman Andrew McAlley said a search of the scene showed that P had recently been manufactured at the factory.

A specialist Auckland P-lab team has been called in to investigate the factory and the area is being guarded by armed police.