The Department of Labour wants more time to decide whether to lay charges over the Pike River mine explosions that killed 29 men in November last year.
The department applied to the Greymouth District Court on Monday for a six-month extension to the time it has to lay charges.
Acting deputy chief executive Lesley Haines says the investigation is one of the most complex the department has done, involving large quantities of records, complex technical issues and significant numbers of interviews.
Under the Health and Safety in Employment Act, the department may lay charges any time within six months of an incident but the act also allows the court to extend that time limit.
The initial six-month period runs out in May.