I’ve found the reference. It’s from the first book written about the accident, entitled ‘White Out’, by Michael Guy in mid-1980, just after the release of the Chippendale Report (pp119,120):
“The search for the flight and voice recorders began straight away. Ian Gemmell from Air New Zealand had promply made a reconnaissance of the area as soon as he reached the crash site. Roped to a Face Rescue mountaineer he had located a part of the fuselage near where the recorders were racked. The mountaineer was the first to see the bright orange voice recorder, but there seemed to be no sign of the flight recorder. They made another sweep along the strewn wreckage, but there appeared to be no trace of it above ground. They decided to dig around the area where the first recorder had been found. It took some time to dig about 30 yards of snow, and they then decided to take a break for tea. By 11pm they were digging again, and within five minutes they were lucky. Gemmell’s spade struck the box, 15 yards away from where they had found the voice recorder. It was an immense relief. Neither recorder appeared damaged and they would possibly yield clues as to why the plane had crashed.”