A 26-year-old man was arrested in Pyrmont and is being questioned over the attack on the Rozelle side of the bridge about 2.45am on Thursday.

A NSW Police spokeswoman said officers from the Leichhardt local area command were called to the Anzac Bridge to reports that a woman had been sexually assaulted.

The woman, aged in her mid 20s, was attacked on the pathway leading up to the bridge that runs beside the old White Bay Power Station.

There are no homes in the area and, while traffic passes just metres away, large trees flank the path in some sections.

The attacker fled on foot before the woman could raise the alarm.

"The woman was treated at the scene by paramedics before being taken to Royal Prince Alfred Hospital. She remains in a stable condition," the police spokeswoman said.

A 26-year-old man was arrested in Pyrmont a short time later and was taken to Sydney City police station, where he was being questioned on Thursday morning. No charges have been laid.

Police set up a crime scene on the walkway early on Thursday morning and forensic police examined the area.

The police spokeswoman said the path was reopened to pedestrian traffic about 5.15am.

Anyone with information should call Crime Stoppers on 1800 333 000 or visit the Crime Stoppers online reporting page.