Police said there were no suspicious circumstances.
Samples of blood, urine and liver taken in an autopsy were forwarded to ESR for toxicological analysis.
While a trace of alcohol was detected, ESR detected about 38mg of BZP per litre of blood.
Toxicologist Leo Schep of the Dunedin-based National Poisons Centre said the blood levels were "surprisingly high, and he may have had a massive dose coupled with post mortem redistribution of the drug".
The manufacture and sale of BZP was banned by Parliament in 2008.
The coroner's finding noted that although there had been no scientific reports confirming death from BZP, ESR had analysed cases where the party pill has been linked with death.
One case involved a fatal motor-vehicle crash, another where BZP was found with alcohol and other drugs, and in four years of testing there were three sudden deaths where BZP was the only drug detected.
The coroner found no evidence Mr Ennis took his own life, and it was "significantly more likely that the BZP was self-administered for recreational purposes".
People taking illegal drugs for recreational purposes "must be warned that the consequences of ingestion are difficult to predict", he said.
Dr Paul Gee, a Christchurch-based emergency specialist, warned anyone who had stockpiled the product to "flush them".
"It is Russian roulette. You don't know what you have got ... and whether it is going to be dangerous."
