By MATHEW DEARNALEY and NZPA

A Christchurch mother who died in a family caving outing at the weekend was not the first victim of an underground stream frequented by tourists and school groups.

Noelleen Merle O'Brien, 40, died at Cave Stream in the Southern Alps about 45km southeast of Arthur's Pass on Saturday after spending three hours in near-freezing conditions with her husband, Peter, and their two children.

Julian, 8, and his 13-year-old sister, Jude, were airlifted to Christchurch Hospital with Mr O'Brien, where they were treated for exposure and then discharged.

Although the police believe Mrs O'Brien was likely to have died from hypothermia, a doctor said last night that some pre-existing medical condition may have contributed.

Her death was the second in the stream in 16 years and happened after the family was warned by Department of Conservation track contractors that the water would be cold and up to chest-deep.

The water temperature in the cave was 5.5 degrees at the time and the air temperature 5 degrees.

In October 1987, a 17-year-old Thai student at Christchurch's St Margaret's College was swept to her death by the current and another girl was lucky to escape by clambering on to a ledge.

A 362m, one-way walk upstream through the cave usually takes 40 minutes to an hour in better conditions, but the O'Briens were unable to climb a waterfall near the halfway point.

Their torch batteries ran out and Mr O'Brien was forced to leave the children in the cave in total darkness and head back for help when his wife succumbed to the cold.

Rescuers waded through water which went above their waists in places before finding the two children and Mrs O'Brien's body.

They piggy-backed Julian out of the cave, but Jude was able to walk.

The family entered the limestone cave, which is below a carpark just off State Highway 73, about 11am. Mr O'Brien emerged about 2.15pm and told DoC contractors Ray and Maree Goldring that he feared his wife was dead and his two children were still in the cave.

The couple radioed the Mt Arthur National Park headquarters and rescuers were flown in from Christchurch.

Mr Goldring said last night from the settlement of Castle Hill, 3.5km from the cave, that they saw the O'Briens on their way down to the cave entrance and warned them about the conditions.

"We warned them it was very cold and the water was going to be chest-deep," he said.

He was also concerned they were wearing normal "street clothing" such as tracksuits and sneakers rather than polypropylene garments.

Despite a sign warning cavers to be prepared and carry spare batteries, he understood they had no replacements when their two torches ran out.

"The man's reaction was that they had warm clothes in the car and were going to change when they got out of the cave," Mr Goldring said.

He and his wife became worried when the family failed to emerge within two hours.

"At about one o'clock we started getting worried. The man came out looking for help about two [o'clock]. He was quite lucid ... "

Peter Dawkings, director of the Castle Hill outdoor pursuits centre, which is used by Christchurch private schools, said he had been through the cave more than 200 times but stopped taking parties through the cave three weeks ago because of the water's depth.

Tourists frequented it in summer, but few went through after April as it was "just too cold".

One waterfall was very narrow and the water pressure would have made it hard to climb, even though it was less than 1m high.

Mr Goldring, who followed Mr O'Brien and a volunteer back into the cave, said it was hard keeping his footing in places because the current was quite swift.

He said the children did not seem aware their mother was dead and the din of the rushing water made conversation impossible in any case.

Senior Constable Chris Lowe of Darfield said the death had been referred to the coroner.

It was likely Mrs O'Brien had died from hypothermia.

But Canterbury Medical Officer of Health Mel Brieseman said it would be very unusual for someone to die from hypothermia in such a short time.

"It's not a common condition in New Zealand, but there are those that develop an allergy to the cold.

"It's an unusual situation because usually cold works as a preserver. It slows the body down.

"It sounds as though she must have had some pre-existing medical condition to have died so quickly."

A neighbour of the family, Rose Haynes, described Mrs O'Brien as a "delightful, outgoing person" who adored her two children.

She was operations manager at the Maori Women's Business Resource and Employment Information Centre in Christchurch.

"I was shattered when I heard what had happened. She had been so excited about the weekend away with the family.

"She was looking forward to going to the mountains, enjoying the fresh air, and enjoying being with the family."