A major search for missing teenager Lily Hooper has entered its third day, with Australian Defence Force personnel joining emergency crews combing rugged bushland in the NSW Southern Highlands.

The 18-year-old experienced bushwalker has not been seen since setting out alone for a hike in Nattai National Park on Wednesday morning, prompting serious concerns from both police and her family.

Lily left her home in Oakdale, south-west of Sydney, telling family members she intended to hike through the national park. Her family raised the alarm when she failed to return home that evening.

Hume Police District crime manager Detective Inspector Benjamin Bowles said Lily had been in contact with relatives during the hike.

“She did have her mobile phone with her, she actually messaged some family members during her hike,” he said.

“However, we’ve [been] unable to contact her on her mobile since that evening.”

Police subsequently located Lily’s vehicle at the beginning of the Bonnum Pic Walking Trail, shifting the focus of the search to an area described by authorities as rugged terrain generally tackled by experienced bushwalkers.

While Lily is understood to be an experienced hiker, police believe it may have been her first time walking the Bonnum Pic trail, although that has not been confirmed.

The search began on Wednesday evening and has since grown into a significant multi-agency operation involving Police Rescue, the NSW State Emergency Service and the Australian Defence Force.

The region’s rescue helicopter has been deployed, while drones operated by Fire and Rescue NSW and the Rural Fire Service have also searched the area from above.

Specialist NSW Police Rescue coordinators are directing Friday’s operation, with air crews, trail-bike riders, SES volunteers and ground teams being deployed across the difficult terrain. Search operations have continued since Wednesday night, although some efforts have been scaled back after dark because of safety concerns.

Despite Lily now being well overdue, police say they remain hopeful she will be found safe.

“She is an experienced bushwalker,” Inspector Bowles said.

“We have not been able to fully confirm what supplies Lily had with her at the time, but we are still hopeful that we will be able to locate her safe and sound.”

Police have also stressed there is currently nothing to suggest Lily’s disappearance is suspicious.

“We don’t have any information to suggest that this is anything other than an overdue hiker,” Inspector Bowles said.

Lily was last seen wearing a shirt and shorts. NSW Police have described her as being about 175cm tall, of slim build, with long blonde hair and blue eyes.

Authorities are particularly keen to hear from anyone who was walking in the Nattai National Park area on Wednesday and may have seen Lily or noticed anything that could assist the search.

Anyone with information is urged to contact Moss Vale Police Station on 02 4869 7899 or Crime Stoppers on 1800 333 000.

Images: NSW Police