The first four hours after Friday's Pike River mine blast was the only time rescuers could have entered the mine, a safety expert says.

Before the service began, 29 candles flickered on a table near the altar surrounded by many unlit candles waiting to be lit by mourners.

It was Pike River CEO Peter Whittall who broke the news to the miners' families this afternoon, that their worst fears had been realised.

Families of the 29 Pike River mine victims who perished in today's second blast fell to the floor screaming and were in "absolute despair", when they were told the news.