"We have found that in the main altararea within the main auditorium the fire has spread across the roof and through the roof voids to other areas of the church, making it very difficult to contain.
"It has been a high fire, meaning it's up at ceiling level and in the roof area."
He said the fire was first spotted by a security guard reacting to a monitored security and fire alarm system, but could have been better contained if the church had a sprinkler system.
"There was no fire visible outside the building when the security guard arrived."
Mr Smith said the guard broke into the church to try to put the fire out.
"But he only got two paces into the building before the fire became too intense.
"From that point the fire's development was rapid, high and very hot," Mr Smith said.
Mr Bootten warned owners of older fluorescent lighting systems to have them checked or replaced as this was the third major fire in the past five years linked to older-model lighting systems.
The minister of St John's Presbyterian Church, the Reverend Ed Masters, said while parish members were devastated at the loss of the church, they were relieved it was not caused by arson.
"It would have been so sad if someone had made a mistake that interrupted their life in such a significant way," he said.
"We believe our city is full of wonderful young people and are so pleased that none of them are implicated in the cause of this tragedy."