Rescuers hopeful missing teenage hiker 'alive and safe'

The search for a missing 18-year-old woman last seen five days ago remains a "rescue" mission, authorities say.

Lily Hooper, 18, has been missing since Wednesday after embarking on a hike in the Nattai National Park in NSW's Southern Highlands.

The Oakdale teen did not return that evening and could not be contacted on her mobile phone, triggering a wide-scale search.

Hundreds of emergency personnel, loved ones and volunteers battled unforgiving conditions across the weekend to no avail.

Detective Inspector Brendan Bernie said a "rescue" remained core to his mission.

"We're treating it definitely as a search and rescue operation and we suspect that Lily is out somewhere in this bushland and nearby," he told reporters on Monday afternoon.

"Lily's an experienced bushwalker. We're relying on that experience to get her through this ordeal and we're still hoping that she's found alive and safe."

Searchers have not found any trace of Ms Hooper since they discovered her car at the Bonnum Pic trailhead on Wednesday night.

Earlier on Monday, a man who identified himself as Ms Hooper's father asked Australians to cross their fingers for his daughter.

"I want to just project positive energy and love, and we're going to find her today," he said in an on-air appeal on Sydney's 2SM Radio.

"I'm just putting my boots on, and I just want everyone to throw love and energy because we've got to go and find her.

The ongoing search includes crews from NSW Police, the Rural Fire Service and the State Emergency Service.

Rescuers have been met with rugged, dense terrain at the national park and cold, wet and foggy weather.

Police have been using a thermal imaging helicopter at night, and earlier revisited any telecommunications data potentially emitted by Ms Hooper's phone.