Flags on government buildings will fly at half-mast on Thursday in New Zealand as Prime Minister John Key expressed his condolences for the loss of 29 miners on Wednesday.

Speaking at a media conference in the capital Wellington, Key said a Commission of Inquiry into the incident should be launched early next week.

Key will also travel to South Island West Coast on Thursday to meet with families of the lost miners and the rescuers who tried to save them.

He said the rescuers had put in an "enormous effort" that could not go unmarked.

Key said the the loss of the miners was a "national tragedy", adding "We are a nation in mourning."

Speaking to those who had lost loved ones in the tragedy, Key said the entire country was standing "shoulder to shoulder" with them during this terrible time.

On behalf of the country, he sent a message of sympathy to all those who had suffered a loss, saying New Zealand was a small country where this type of tragedy affected so many.