A New Zealand judge has convicted a coal mining company of nine health and safety violations over a 2010 explosion that killed 29 miners.

Judge Jane Farish on Thursday found Pike River Coal had failed to ensure the safety of its workers.

The now-bankrupt company didn't defend itself against the charges. An earlier government investigation found the company ignored 21 warnings that methane gas had accumulated to explosive levels in the mine and it was exposing miners to unacceptable risks as it strove to meet financial targets.

The company will be sentenced in July. Each of the charges comes with a maximum penalty of 250,000 New Zealand dollars ($211,000).

Former chief executive Peter Whittall has pleaded not guilty to 12 charges. His case has yet to be heard.