Police are concentrating on a small area of Arthur's Pass, as they continue their search today for missing British tourist Elizabeth Margaret Thomson.
Ms Thomson, 55, of Kent, England, has not been seen since Saturday, and police search and rescue teams are back today combing the area.
Ms Thomson failedto return from a day tramp in the Arthur's Pass National Park.
Groups of searchers with dogs have been scouring the area since the alarm was raised on Sunday and they headed back out again this morning.
Trampers found her backpack on Sunday on the Mount Aicken track at about 1700m above sea level.
Head of the rescue effort Sergeant Peter Summerfield said the search teams, including a helicopter, were going to concentrate their search on an area in about a 400-500m radius from where Ms Thomson's pack was found.
Mr Summerfield said they would be looking for signs of the direction in which Ms Thomson could have headed.
"But it's just difficult. If she's in the open we think we would have found her, but if she's out of sight in the scrub or tussock, she's quite a small lady so she's not going to have a very big profile. We could be within five feet of her and miss her," he told National Radio this morning.
The terrain was quite rugged and the bush and scrub was waist high in parts and parts of the bush were thick and impenetrable, he said.
Ms Thomson was a keen photographer, and Mr Summerfield said they were working on the theory she had left her pack to take photographs away from the track.
"And so we're thinking . . . either: A -- she can't find her pack and tried to find an alternate way down, or B -- she's gone off to take a photograph and something happened to her, she slipped and has fallen out of sight. But we don't know."
The area from where the pack was found, to the summit of Mount Aicken was not marked in any way, Mr Summerfield said.
"So people who are not familiar with it could go off track or off the route quite easily and then get stuck trying to find their own way down or find their way to where their pack was."
He said conditions had improved since last night, when it rained heavily.
