The blast was so powerful it shifted bricks in the outer walls of the flat and sent window glass flying across a Hamilton street.

Inside the Frankton home, a badly burned man rushed about trying to hide chemicals as skin flapped from his scorched arms and flaming curtains fluttered through the windows.

The explosion, at 21 Queens Ave at 4.45pm on Thursday, was apparently caused by a cannabis oil-making attempt that backfired.

The man was in hospital yesterday, just six days after he shifted into the flat.

Neighbour Katherine Wright said she was at home when she heard a bang that sounded like someone falling through a window.

She and other neighbours ran to the unit, which was on fire.

"I called out 'is everyone alright, is everyone okay?"'

There was no response. She entered to find a badly burned man in his mid to late twenties trying to put out the blaze.

Window frames had popped out, the kitchen was blackened and paint was peeling from the ceiling.

Neighbours moved a gas bottle out of the unit and tried to quell the fire with a hose. Then they turned it on the man's burned face and arm to cool him.

The man's chest, covered only by a singlet, was also red. His girlfriend, who was also in the unit, was less badly burned.

Ms Wright said the man panicked. There were spray cans, paint cans and pots around the stove.

"He was saying 'I've got to get this stuff out of here'."

There was a strong smell of chemicals. Ms Wright believed the man had been making "oil" on the stove.

Police found cannabis and chemicals when they arrived soon after.

Glass, a yellow curtain, pieces of window frame, a blackened pot, a knife and plastic containers lay scattered across the lawn yesterday. Debris lay in neighbours' gardens.

Police, forensics staff, photographers and scene guards were at the flat.

A policeman confirmed that charges would be laid. However, the burned man was too badly injured to be interviewed.

"He's not going to die," the officer said. "But ... he won't be going anywhere in a hurry."