Search resumes for missing Sydney girl

The search for missing Sydney teenager Cassie Olczak will resume with police unable to find any trace of the missing girl last seen on Sunday.

Helicopters, trail bikes, the dog squad and about 40 volunteers started scouring the Royal National Park about midday on Wednesday.

The search was triggered after CCTV vision was discovered of the 16-year-old at Waterfall Station about 7pm on Sunday.

Cassie went missing after visiting friends a day after returning to Australia from a trip to see her father in Abu Dhabi.

Police wrapped up the search just before dark on Wednesday but said they were hopeful they would find her when the search recommenced on Thursday morning.