Gases from a P lab in a cave on the Coromandel Peninsula are believed to have been responsible for killing two men cooking the drug.
The bodies of the men were recovered from an old mine shaft on the 309 Rd between Whitianga and Thames yesterday.
Police said they had taken "material" from the cave to be analysed but would not comment on whether there was a P lab or an explosion.
Police have also not revealed who alerted them to the bodies on Saturday afternoon.
Firefighters were on alert for chemicals or flammable liquids when they were called to the scene about 9.30pm on Saturday.