When Miller arrived he was excited to tell O'Neill he'd managed to store two components of oxy-fuel, oxygen and acetylene, into one LPG bottle.
The two are normally kept in two bottles, each of which has a regulator, the two components meet at the welding torch causing the chemical reaction used to cut or weld.
"He didn't realise he had made a bomb," O'Neill said.
Howard Miller was a father of three.
Police Detective Senior Sergeant Callum McNeill said there had been speculation on social media that the house was a P lab, but that was not correct.
"The investigation is ongoing and ultimately the matter will be considered by the Coroner, however initial indications are that Mr Miller had gone to a friend's house to help weld an exhaust onto a car," he said.
"Police's thoughts are with Mr Miller's family at this difficult time."
Leth, a 36-year-old photographer who now lives in Brisbane, said Miller was his best friend for the past 10 years.
Both men came from South Africa, but met in Auckland. Miller's father, Baptist Pastor Herbert Miller, and other family members came to New Zealand first and Howard followed.
Howard and his wife Abigail were regular church-goers.
"He was very religious, he loved God," Leth said.
The couple have three children aged between about 14 and 8.
Howard played in a social soccer team on Thursday nights.
