AN ice shelf has reportedly collapsed on the Fox Glacier, trapping four tourists on the west coast of New Zealand's South Island.

St John Ambulance spokesman Ian Henderson told NZ media they were alerted to the collapse shortly after 5pm (1600 AEDT) on Thursday and had received information that there were at least four people trapped, but had no details on ages or genders.

The Solid Energy Rescue helicopter with paramedics on board and an ambulance from Fox Glacier had been sent to the glacier, he said.

Police said it was understood that two people were trapped under the ice and the Fire Service was in attendance.

No further details were immediately available.

TV3 News reported the collapse was outside the roped area of the glacier.

In 2007, the Department of Conservation said that almost a third of the 600,000 visitors to the west coast glaciers ignore warning signs and enter danger zones.

In February that year, a tourist standing beside an ice cave at the terminal face of the Franz Josef Glacier was injured when the roof collapsed. He had walked past signs warning of the danger of falling ice.