Police and the Department of Labour will soon decide on whether to lay criminal charges over the Pike River mine tragedy.

As a royal commission of inquiry is being held, police and the department responsible for workplace safety have said that their investigations into the 29 deaths in the mine last November were well progressed.

Police say they are about three-quarters of the way through their investigation after carrying out more than 260 formal interviews.

"Some of [the interviews] have lasted anything up to three or four days," Detective Inspector Peter Read told the Herald.

Mr Read said police expected to receive the views of an expert panel on mining next month, which would than be compared with the opinions detectives had reached. It would likely be October when police released their decision on whether any corporate or individual was to be charged.

The Department of Labour says it is about 80 per cent through its investigation.

Day six of the commission of inquiry yesterday heard that New Zealand had no approved codes of practice for underground mining. Department of Labour workplace health and safety policy manager James Murphy said there was no need for such a code for underground coal mining.

The lack of dedicated mines inspectors has been criticised in the inquiry. Mr Murphy said the ratio of two health and safety inspectors for four underground mines in New Zealand had been assessed a lot. The ratio was found to be adequate, he said.