A fourth underground explosion has rocked the Pike River Mine in New Zealand's south island, where 29 miners earlier perished.

Police say there were no injuries when the gas explosion occurred just before 2:00pm local time on Sunday.

People who were working close to the mine entrance were moved to a safer distance.

Police say this fourth blast demonstrates the ongoing volatility of the site.

Last week, 29 men died last week when a series of explosions ripped through the underground mine, prompting prime minister John Key to say he will recommend a royal commission into the tragedy.

Mr Key says a royal commission is the most powerful investigation available under New Zealand law.

It would be chaired by a judge and have the power to compel witnesses to testify, leaving no stone unturned in the search for the cause of the blasts.

Unions have backed the call for the inquiry, but say it should be larger, including representatives of the lost workers and their families.

Meanwhile, preparations are under way at the picturesque Omoto Race course just outside Greymouth for a crowd of up to 7,000 mourners expected at a National Memorial Service on Thursday afternoon.

Emergency crews have been unable to recover the bodies miners as the mine remains flooded with explosive methane gas.

Twenty-four New Zealanders, two Australians, two Scots and a South African died in the mine near Greymouth.

One New Zealand news agency is reporting that today's explosion blew away some of the infrastructure on top of the shaft.