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Stephanie Simpson (32) disappeared in Mt Aspiring National Park over the weekend after telling friends she planned to hike to Brewster Hut and then move on to Blue Pools.
Brother-in-law Sam Hazelton told the New Zealand Herald her family in the UK were "hoping upon hope" that she will be found.
"You can't imagine, it's pretty awful. The hardest thing, I think, is just the not-knowing.
"Everybody's just hoping. You go to bed thinking: 'What can we do?'
"And then you wake up, or keep checking your phone to get an update...the hard thing is the delay and then nothing comes."
Ms Simpson, from Essex, was reported missing around 8.45am on Monday when she failed to keep an appointment.
Search and rescue teams have been scouring the rugged terrain with teams on the ground, dogs, helicopters and drone with thermal imagery.
Mr Hazelton said the search was to pick up again at daylight today.
Police said this morning the search would resume at 8am and involve 16 LandSAR members and two dog teams.
Searchers would today be concentrating their efforts in and around the Pyke and Fantail Creek areas.
"There's been a huge amount of effort. I can't say how much everybody appreciates everything that's gone into the search," Mr Hazelton said.
"But there's not been a huge amount of progress overnight."
Mr Hazelton pleaded that anyone with information that might help to find Stephanie to come forward.
"The family are just at their wits' end, they're trying really hard to cope.
"If there's anyone else that has any information, or has seen her and can support them narrowing down a search area, let the police know."
Police continue to appeal to hear from anyone who stayed in the Cameron Flat area on Friday and saw or spoke to Ms Simpson, or anyone who may have seen her on the track from Fantail Falls to Mt Armstrong, or in Makarora Valley.
Anyone with information should call 105 and quote file 200211/4896.
"The team said they're searching for somebody who's alive, but they also have asked a number of questions," Mr Hazelton said.
"She's an extremely determined, strong-willed person with experience and I think that's giving them the hope to continue to search and find her."
Stephanie was an extremely fit, experienced tramper, her brother-in-law said.
"She's someone that's done triathlons, running races, marathons, she's really sporty and picks up new skills quickly."
Passionate about tramping, she had hiked for many years in the UK, Australia, and recently, New Zealand.
The last time the family heard from Stephanie was when she messaged a family WhatsApp group on Thursday morning that she was going tramping for the weekend.
"It was a normal weekend tramp. That's what she's there for, just to make the most of that free time," Mr Hazelton said.
"She has a real interest in exploring in the outdoors in the environment, she loves new challenges and new adventures."
It is believed she headed out to Mt Aspiring National Park after work on Friday and was expected to return on Sunday.
She was carrying a day pack and did not take a search and rescue beacon with her.
Mr Hazelton wanted to thank everyone involved in the ongoing search.
"We want to give a huge thank you to everybody, whether it's friends, volunteers, the specialist teams.
"We appreciate the unbelievable efforts that are gong on and the work that's going into this and we can't thank them enough."