As the new work week begins, the NZ Transport Agency is urging people to use public transport instead of driving over the Auckland Harbour Bridge.

This comes as the agency announced that the design and fabrication of the permanent strut to repair the damaged bridge is now well advanced.

The Grade 350 L15 steel for the strut has been sourced and manufactured by NZ Steel and delivered to fabricators in Whangārei and will be transported to Auckland by truck.

This will replace the one that was damaged when 127km/h winds blew a truck into the bridge superstructure last Friday.

The new strut will return the bridge to its full load capacity, and two of the bridge's eight lanes will remain closed until it is installed.

"Teams are working at pace to design and peer review the replacement part for the permanent solution and to manufacture and plan for installing it," the agency's transport services general manager, Brett Gliddon, said.

"Fabricating the 22.7-metre strut and getting it into position is the quicker part of the process. It will take several weeks before it is tensioned up as a load-bearing part of the bridge superstructure.

"The bulk of this time is in the design, calculation and external peer review of the new strut in relation to how it will impact on the performance of the whole bridge. This includes careful calculations on how to re-balance its load-bearing function."

The installation will require closure of southbound lanes and will take longer than the temporary fix.

A date for the installation is still not known.

Although the new strut will not exactly match the original, the agency was working with a consultant conservation architect to ensure it would fit appropriately with the landmark status and heritage values of the bridge.

"The aim is that once it's installed and had a coat of paint, it will blend in with the rest of the bridge superstructure and the subtle differences will go unnoticed," Gliddon said.

The agency is reminding motorists the bridge is operating only at 75 per cent capacity and two lanes are closed.

Until the permanent repair, Gliddon urges people to change their travel patterns and use public transport.

Motorists are advised to avoid using the bridge if possible, travel during off peak hours, use the Western Ring Route on SH16 an dSH18 or leave their vehicles at home and take public transport to reduce congestion and delays on the motorways.

Last week started with a 50 per cent drop in the number of vehicles crossing the bridge compared to the previous week, but the number was only about 22 per cent less as the week went on and two lanes reopened.

"We are asking motorists to avoid using the bridge, not just to reduce congestion and delays. The bridge and the clip-ons are safe to use, but our ability to change the lanes configuration for peak time traffic is constrained and there is a heightened risk if there is a minor crash or breakdown on the bridge," Gliddon said.

"Any disruption on the bridge, even for a short time, has a swift knock-on effect across the motorway network and local city streets."

The agency said there were plenty of seats on scheduled bus services from the North Shore and travelling by bus on the Northern Busway could be quicker than by car on the motorway.

No overweight or over-dimension vehicles are currently allowed on the bridge.

A bus priority lane will be implemented on the SH1 northbound motorway for buses travelling during peak times. Motorway signage will show that the left-hand lane is prioritised for buses.

To allow traffic in the bus lane to keep flowing smoothly, drivers are asked also to be careful not to block intersections and let buses through to the motorway.