A massive overhead road sign falling and crushing a woman's car as she drove along one of Melbourne's busiest freeways was "a very extremely rare event", according to one of the directors of Victoria's major road projects authority.

Key points: Wind did not cause the sign to fall, authorities say

Wind did not cause the sign to fall, authorities say The Major Road Projects Authority is investigating

The Major Road Projects Authority is investigating The freeway exit near the sign was recently upgraded

The woman was driving on the Tullamarine Freeway when the five-by-four metre sign over the inbound exit for Bulla Road and Bell Street came down on her car, shortly before 6:00pm on Tuesday night.

"There were a lot of other cars on the road, she's the only one that's been hit by the sign," said Acting Sergeant Jason Lane from Fawkner Highway Patrol.

"It is unusual," he said. "I haven't come across it before."

She was taken to the Royal Melbourne Hospital with neck and hand injuries and is in a stable condition.

The front and back of the woman's car was smashed when the sign fell. ( ABC News )

Acting Sergeant Lane said wind was not a factor and that Victoria Police was looking for any dashcam footage of the incident.

'Thorough investigation' launched

The Bulla Road exit was recently upgraded as part of the CityLink Tulla Widening project.

Graeme Chambers from Major Road Projects Victoria (MRPV), the statutory body which oversees the delivery of new roads, freeway upgrades and widenings, said the incident would be investigated.

He said a team of experts from the MRPV, VicRoads, Transurban, and other consultants were investigating.

"Clearly something has gone wrong and we want to find out why that sign has fallen," he said.

"It's an extremely rare event. I've had 30-plus years' experience in the industry as an engineer and I'm not aware of such an event happening before.

"We will not stop until we find out the cause."

Sorry, this video has expired Sign falls and crushes car on Tullamarine Freeway

Mr Chambers said it appeared the problem was with the sign's connection with an overhead gantry.

"The gantry would have been designed by an engineer, it would have been proof engineered, it would have been independently reviewed and would have been quality certified, so we're going through all that documentation as part of the investigation."

He said a sign adjacent to the one that fell had been checked and did not pose any danger.

Freeway safe to drive on, VicRoads says

VicRoads' deputy chief executive Robyn Seymour said the stretch of freeway had been under the road authority's management when it was handed over by MRPV 12 months ago.

"This is a brand new road, this is not what you would anticipate on a brand new road," she said.

She said VicRoads had inspected the sign after it was built.

Ms Seymour said VicRoads checked the freeway network twice a day for hazards, conducted six-month checks on all signs, gantries and bridges and did a thorough condition review of roads every two to four years.

She reassured motorists that the freeway was safe to drive on.

An investigation into how the sign could have fallen is underway. ( ABC News )

"It is our number one priority to have a safe network," she said.

"We would not have the freeway open if we believed there were risks to the community today."

A Transurban spokesperson said similar signs installed as part of the CityLink Tulla Widening project were checked overnight Tuesday.

"Safety is of paramount importance to us and we will continue to work in collaboration with VicRoads, Major Road Projects Victoria and CPB Contractors as part of the investigation," the spokesperson said in a statement.