Another year of roadworks and delays on a $268 million upgrade of Auckland's Southern Motorway between Manukau and Papakura is a kick in the guts to motorists, says the Automobile Association.

The work has been pushed out to the end of next year after changes to the initial plans, including the design of the Takanini northbound on-ramp and the need to replace and not just widen two bridges over the Pahurehure Inlet.

The Southern Corridor Improvement project, which began in October 2015, is extremely complex and taking place within a highly constrained and narrow motorway corridor, said New Zealand Transport Agency senior project manager Chris Hunt.

The fact that they think they can just slip another year of this under the carpet is infuriating and insulting

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AA principal adviser of infrastructure Barney Irvine said from a congestion perspective, the works at Takanini are one of the most painful points on the Auckland network.

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"They make life a misery for commuters, for people visiting Auckland for work, and for people trying to get out of town on weekends or holidays, and we hear about it constantly from our members.

"So any news that the project's been delayed will be a kick in the guts for Auckland motorists. There may be sound technical reasons for the delay, but it will still be a bitter disappointment," Irvine said.

Bombay resident Catherine Fuller said there were still signs on the motorway a few weeks ago saying 'New lanes completed by the end of 2018'.

"My husband sits in that traffic day after day for up to 1.5 hours each way on a route that without traffic would take 30 minutes.

"The fact that they think they can just slip another year of this under the carpet is infuriating and insulting," Fuller said.

The Southern Corridor Improvements project covers the stretch of Southern Motorway from the SH20/SH1 connection at Manukau down to Papakura.

The project includes additional lanes in both directions, upgraded Takanini Interchange and a 4.5km shared-use pedestrian and cycle path.

Hunt said the entire project is expected to be completed and fully opened by the end of next year, saying an additional southbound lane between Hill Rod and Pahurehure Inlet has already been opened and the rest of the project will be completed in phases and open from early next year.

He said the initial design of the Takanini northbound on-ramp had to be updated from a diamond configuration to a loop, which will take longer because of additional structures and working within a more constrained site.

"This also has a knock-on effect for other parts of the project, mainly south of the Takanini interchange," Hunt said

The two bridges over the Pahurehure Inlet carrying northbound and southbound traffic were initially planned to be widened, however more detailed investigation discovered that the supporting piers should be fully replaced to ensure the bridge remained structurally sound for years to come.

Hunt said work in this coastal marine location is complex and needs to be staged in order to keep traffic flowing and to minimise the environmental impact on the Pahurehure Inlet.

A meeting is being held in Pukekohe tonight by the NZTA as part of regular consultation to update people on the works.