Roads and Maritime Services closed the road for two days this month for investigations after a Thirroul couple was injured by a 30cm piece of falling rock while driving up the pass.

RMS Southern regional manager Renae Elrington said geotechnical experts had identified a zone, high on the escarpment, where the falling piece of rock likely originated.

The zone was on land held by the National Parks and Wildlife Service and was higher than some existing sections of mesh fencing that had been installed on the escarpment to catch smaller pieces of falling rock, Ms Elrington said.

Investigators will report back to the RMS early next month on their findings, and options for improving safety on the 1km thoroughfare.

"A possibility we could be looking at is a different design of fence," Ms Elrington said.

"Our question to [geotechnical experts] is, what would be appropriate to this area?"

Thirroul man Colin Bird was hospitalised for three days from January 21, when a large piece of rock fell from the rock face onto his Land Rover, smashing the windscreen and bouncing "agonisingly" onto his lap.

In the aftermath Mr Bird and his partner Suzanne Latham, who also suffered cuts and whiplash in the accident, called for signs warning of the danger of rockfalls to be installed on the pass.

Ms Elrington said the couple's accident was rare, and was the only known occurrence of a rock from the pass falling onto a car.

Finding space to install a sign on the narrow road could be challenging, but would be considered, she said. According to a spokesman, the RMS inspected sections of the northern escarpment, including Bulli Pass and Mount Ousley, twice a week as part of regular maintenance.

The spokesman said work carried out on the pass in the past three years included two major wall and embankment repair projects close to the road; replacement drainage pipes; replacement guard rails; rock bolting and shotcreting (where concrete is sprayed and reinforced with bolts and steel mesh), removal of unstable rocks and mesh fencing in "priority locations".

Mount Ousley was also recently closed for investigations, while Macquarie Pass will close today between 9am and 3pm for similar work.