New evidence to be presented to the Pike River Royal Commission shows a Japanese mining expert quit the mine because he was worried it would explode.
The evidence will be heard as the inquiry investigates what went wrong in the Pike River mine, leading to the disaster almost a year ago in which 29 men died. The hearing will start next week.
The written evidence of Masaoki Nishioka has been seen by Radio New Zealand News ahead of the hearings next week.
Mr Nishioka had 40 years experience of high pressure water mining but says he left Pike River after only three months because he became too frightened to go underground.
He catalogues serious design faults with the water system, the coal faces being mined and the system for draining away explosive methane gas.
The Engineering, Printing and Manufacturing Union says Mr Nishioka's evidence raises many important questions.
Assistant national secretary Ged O'Connell says the evidence will be very significant.
However, he says people will want to know why Mr Nishioka did not report the mine's problems before he left, a month before the mine began exploding on 19 November last year.
Mr O'Connell says the union will be scrutinising Mr Nishioka very closely when he appears so his whole story comes out.