The driver of a truck that brought Sydney traffic to a standstill after it got stuck in the M5 East tunnel has told police the incident occurred when he dropped his glasses.

In reaching for them, he said, he hit a button or lever that lifted the back of the truck into the tunnel ceiling.

The accident, which closed the tunnel, was one of two overheight truck incidents to disrupt traffic on Thursday morning.

Just before 10am, the Sydney Harbour Tunnel was closed southbound to allow a truck that had approached the tunnel to reverse into a breakdown lane. The tunnel was subsequently reopened with no damage to the tunnel.

Meanwhile, the motorway approach to the city from the south-west - the M5 East tunnel - remains closed after the damage done to the tunnel ceiling shortly after 8am.

A spokeswoman for the Transport Management Centre said the truck was not overheight when it entered the tunnel, but its trailer had lifted up.

"Somehow the trailer has hydraulically lifted up. Something has happened that has caused the trailer to rise up," the spokeswoman said.

"Technically the truck itself wasn't actually overheight but when it raised up it became overheight."

NSW Police are investigating the incident.

There is no forecast on when the M5 East tunnel would be repaired, according to the Transport Management Centre, and buses through the area are also delayed as traffic spills out onto alternative routes.

The spokeswoman said there appeared to be a lot of glass on the ground in the tunnel, with substantial damage to the ceiling infrastructure. The truck remains in the tunnel.

All westbound lanes of the M5 tunnel were open but the citybound tunnel was closed.

Buses are delayed across Sydney.

Traffic on the M5 motorway is returning to normal. But on alternative routes through the south-west of the city, there remains substantial congestion.

The Transport Management Centre says buses are delayed through Kingsgrove and nearby suburbs by up to 100 minutes.

Because buses that move through Kingsgrove then run on other routes, there are knock-on delays for services as far as Drummoyne, Parramatta, Bondi Junction, Blakehurst, East Hills and Liverpool.

M5 traffic kills my life �� I just want to get to the airport. pic.twitter.com/mjUjCklkPM — Vi (@veetrannnn) November 13, 2013

The incidents are just the latest in a steady string of overheight trucks that have been stuck in tunnels, worsening Sydney's already crippling peak hour traffic.

This is despite Roads Minister Duncan Gay introducing new fines and suspensions for trucks damaging tunnels.

Penalties of $2200 and the loss of six demerit points apply for driving overheight trucks into tunnels. But from June it became easier for the government to pursue the companies that own the trucks for the cost of repairing the damage.

In addition, trucks that get stuck will have their registration suspended for three months.

Mr Gay will hold a press conference at 1pm to talk about the incident in the M5 East tunnel.

- with Amy McNeilage