Restrictions on most trucks using Auckland Harbour Bridge's newly bolstered clip-on lanes will be lifted before Christmas, but the Transport Agency says it will get tough on over-weight vehicles.

It will fit number-plate recognition cameras to spot trucks with loads of more than 44 tonnes on the clip-ons.

Although it will not be able to use these to prosecute, it will send warning letters and pass details of frequent offenders to the police, so trucks can be pulled over and weighed at other locations.

The weight limit has been imposed to extend the economic life of the clip-ons beyond 20 years.

Vehicles weighing 13 tonnes or more have been banned from the outside lane of each clip-on since mid-2007, after engineers warned of a potential for "catastrophic failure" in a worst-case scenario of an end-to-end early morning traffic jam of trucks carrying containers from the port.

The warning sparked a structural strengthening project in which 920 tonnes of extra steel have been welded into the hollow box girder clip-ons at a cost of $86 million, up from an initial estimate of $45 million.

With the project drawing to a close, the agency says it is prepared to allow trucks of up to 44 tonnes full use of the clip-ons from early December, subject to annual reviews of the bridge's loading capacity.

Although permits can be obtained for over-weight loads, these are restricted to the main bridge, and then only at certain times.

Freight operators have also offered to ensure trucks keep 20m apart to spread their weight, putting off the day when the clip-ons reach their "critical live load capacity".

The agency indicated last year that heavy trucks would be unlikely to be allowed back on the outer clip-on lanes, despite the extra steel being welded to the girders, which were joined to the main bridge in 1969.

But northern highways manager Tommy Parker said yesterday that engineers had decided an even distribution of trucks across both lanes of each clip-on would minimise metal fatigue, as well as making it safer for them to get into the lane needed to leave the motorway at Onewa Rd.

While the ban has been in force, northbound trucks have had little room after crossing the bridge to manoeuvre into the exit lane.

Agency traffic and safety manager Kathryn Musgrave said that by introducing "soft" measures to delay the point at which the bridge reached its capacity load, the organisation hoped to avoid later restrictions later which would reduce its economic use.