The normally frantic bridge was briefly closed to traffic and a minute's silence held at 9.27 last night, exactly 40 years after the bulk carrier Lake Illawarra slammed into the structure, bringing down a 127-metre section of roadway.

Four cars plunged into the River Derwent, killing five people, alongside seven crew onboard the Lake Illawarra.

Last night, with the lights on the bridged dimmed, Hobart Lord Mayor Sue Hickey and Clarence Mayor Doug Chipman unveiled a small plaque above the point of impact.

The two mayors were also on hand to open an exhibition commemorating the bridge collapse at Rosny.

On display was Frank Manley's green Monaro, which was famously pictured teetering on the edge of the missing bridge span.

Mr Manley said he would never forget the events of 40 years ago.

"My wife said, 'put it in reverse, put it in reverse'," he said.

"I said, 'stuff reverse, get out' because the car was see-sawing."

"We only had to go another inch, and the point of balance would have changed and we definitely would have went [over]."

Emergency Services Minister Rene Hidding said the bridge disaster had a profound impact on life on Southern Tasmania until the bridge was reopened in October 1977.

"This had significant and wide-ranging economic and social impacts on Hobart and particularly the Eastern Shore, demonstrating the importance of the Tasman Bridge," he said.

"The true legacy of the disaster is how Tasmanians pulled together in the days, weeks and months later.

"As they were then, our thoughts today are with the families of those who died and with the survivors, rescue workers and all affected by this tragic event."