The search for a missing 18-year-old woman is resuming in rugged terrain in a New South Wales national park, as her family wait anxiously for clues about her whereabouts. 

Lily Hooper, 18, left Oakdale in Sydney's southwest at 8am on Wednesday after telling her family she was going hiking in nearby Nattai National Park. 

Ms Hooper did not return in the evening and could not be reached, despite having sent messages from the park earlier in the day.

A multi-agency search has since found her car near the Bonnum Pic trailhead but police, SES and fire services have not found any further signs of Ms Hooper.

Fresh teams were sent out from 7am on Saturday to continue searching for the teenager. 

Hume Police crime manager Detective Inspector Benjamin Bowles said on Friday authorities were concerned for Ms Hooper's safety.

However, he remained optimistic and said Ms Hooper was an experienced hiker.

'We have not been able to fully confirm what supplies (Ms Hooper) had with her at the time but we are still hopeful that we will be able to locate her safe and sound,' he said.

'We don't have any information to suggest that this is anything other than an overdue hiker.'

Helicopters, drones and specialist rescuers including trail-bike riders have been involved in the search. Defence force personnel also joined efforts on Friday. 

Bonnum Pic – a gruelling 17km return journey – features both on-track and off-track trails, dense bush and rock scrambling, and rewards walkers with impressive views from a ridge that juts into the Wollondilly Valley. 

Speaking to the Daily Mail on Friday afternoon, Lily's uncle, Trent Brideson, said the family had received no new information. 

He said her parents, Kirrily and Jim Hooper, were 'understandably not coping very well'.

Lily's aunt, Sheridan Munro, shared on Facebook: 'My beautiful niece Lily Hooper went on a hike in the Southern Highlands and hasn't returned. We are desperately trying to find her.'

Mr Brideson said Lily's family has been unable to join the search, saying 'only authorities' were permitted to track her last-known movements into the park. 

While it's understood police believe this is a hiking-related incident, a local bushwalker said they used to visit the park but 'got too scared when I noticed a few unusual people and I ended up stopping'. 

Lily, who turned 18 in December, is a recent graduate of Campbelltown Performing Arts High School, where she was a popular student.

She is extremely fit and, according to her public social media, returned from a Muay Thai martial arts training camp in Thailand just weeks ago, describing it as 'epic'.

The teenager is from a close-knit family, and regularly posts pictures of herself with her sisters and cousins.

NSW Premier Chris Minns told reporters he was 'praying for good news'.

'Our heart goes out to that young walker's family. We know this must be heartbreaking and incredibly anxious,' he said.

Lily is described as being of Caucasian appearance, about 175cm tall and of slim build, with long blonde hair and blue eyes.

She was last seen wearing a shirt and shorts.

Police are particularly keen to hear from anyone who walked the Bonnum Pic walking trail on Wednesday or who may have information about Lily's whereabouts.

Anyone with information is urged to contact Southern Highlands Police Station or Crime Stoppers on 1800 333 000.