Breakdowns and crashes are creating headaches for Auckland commuters already dealing with a range of roading changes alongside the start of term 3.

At 8.30am the NZ Transport Agency said there were patches of heavy traffic citybound on the Northern, Southern and Northwestern motorways.

Meanwhile a breakdown was blocking the right northbound lane on State Highway 1 in the Victoria Park Tunnel, and a truck breakdown was blocking a citybound lane from the off-ramp to Mt Wellington Highway.

Earlier there was a truck breakdown partially blocking lane two northbound on the Harbour Bridge, and a crash on Ash St, blocking the left lane outside the racecourse.

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Students of 36 central city schools could need to travel on a new school bus route, a public transport service, or a combination of both.

Most routes would be given new route numbers, while some bus routes were changing to remove differences between the morning and returning afternoon routes to make them easier to understand and use.

Some routes were changed due to low patronage in streets along the current route.

Services have been removed in some areas with good public transport options, while services have been introduced in other areas where public bus routes had been altered and no longer provided for students' travel.

The school bus changes were part of a wider programme called the New Network, which was rolled out on July 8 with many buses arriving more frequently every day of the week, and better access to popular destinations.

Auckland Transport's network development manager Anthony Cross said they were creating more options for people to travel where they wanted, when they wanted.

"While it may take some time getting used to the changes, we are confident it provides public transport users with more freedom and more travel options."

The NZ Transport Agency announced it would be increasing the variable speed limits on sections of State Highways 16 and 20 around the tunnel to allow maximum speeds of 100km/h in certain driving conditions.

Variable speed limits meant that the maximum legal speed limit could change between 30km/h and 100km/h and be adjusted for the road and driving conditions.

NZTA acting system design manager Kevin Reid said speed limits were likely to be reduced during peak periods when there were typically high traffic volumes and in the event of incidents, crashes, maintenance work or extreme weather conditions.

The first phase would happen on SH16 between St Lukes and Rosebank and on SH20 between Maioro St and Dominion Rd.

The second phase, on SH16 between St Lukes and closer to the central motorway junction, would follow at a later point once further work on signage and system requirements had been finalised.

"The Transport Agency takes adjusting speed limits very seriously and we have appreciated people's patience while the necessary investigations, consultation and legal processes were completed," Reid said.

Speed limits in the Waterview Tunnel and its approaches would not change, remaining at 80km/h maximum to manage the higher risks associated with an enclosed tunnel environment.

Northern Motorway
Northbound: Free flowing.

Citybound: Heavy between Tristram Ave and Onewa Rd.

Southern Motorway
Southbound: Free flowing.

Citybound: Heavy between Drury and Takanini, heavy between Mt Wellington and Newmarket, and truck a breakdown is blocking the left right turn lane from off ramp to Mt Wellington Highway.

Northwestern Motorway
Westbound: Free flowing.

Citybound: Heavy at Royal Rd and between the Causeway and the Northern Link.

Waterview Tunnel Southbound Free flowing.

Southwestern Motorway
Northbound: Free flowing.

Southbound: Free flowing.

Waterview Tunnel Northbound​ Free flowing.