He was also found with anabolic steroids, prescription medicines and $178,000 in cash.
A later search in a Hamilton city storage unit revealed the group's pill press - with a capacity of producing up to 5000 pills an hour - and large quantities of unidentified powders.
Detective Senior Sergeant Nigel Keall, the Waikato crime services supervisor, said the substances were pressed into pills and sold throughout the country for considerable profits.
He said medical experts were concerned the drugs were being sold with amateur advice as to their use and dosage with little regard for consequences.
"Clients also received guidance around drug-testing and what substances could be used to avoid positive test results for those involved in high-level sport."
Police also searched homes in Auckland and Christchurch revealing more unidentified pills and performance and image-enhancing drugs. In total they recovered 21kg of 4-MEC.
"This is sufficient to manufacture over 200,000 pills at a standard street value of $40 each," said Mr Keall.
Two Auckland men have been charged with offering to supply 4-MEC and police say further arrests are likely pending results of scientific analysis of the unidentified powders and prescription medicines.
Jethwa was remanded on bail.
