The cost of Wellington's smart motorway rose from $55 million to $81m, OIA documents have revealed. (File photo)

The price tag for Wellington's smart motorway jumped by $25 million because design elements were changed after the project was signed off.

The motorway – a section of State Highway 1 between the Terrace Tunnel and Johnsonville, and SH2 between Ngauranga and Petone – was signed off at $55.8m, but ended up costing taxpayers $81m.

Documents released under the Official Information Act reveal a chief reason the New Zealand Transport Agency (NZTA) poured millions more into the project was to expand the technology used in the design.

MONIQUE FORD/STUFF The motorway was intended to reduce travel times and improve reliability between the Terrace Tunnel and Petone. (File photo)

Early completion of the first section of cycleway between Petone and Ngauranga also added to the project cost, as did additional road surface works and the use of a long-life porous asphalt.

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The motorway is the first of its kind in New Zealand. It features a computerised system that sets changeable speed limits based on predicted traffic flows.

MONIQUE FORD/STUFF It works on a computerised system which sets changeable speed limits based on traffic flows. (File photo)

An NZTA spokesman said after the project got underway, it was decided the technology needed to be expanded at the merge point with the Wellington Urban Motorway – a section of SH1 from the Ngauranga Gorge to the Wellington CBD – to better manage traffic there.

The extra equipment also allowed for better traffic flow northbound into Horokiwi, the spokesperson said.

The expansion required a new gantry to be constructed north of the SH2 on-ramp, and an upgrade of the gantry at Horokiwi. A gantry is an overhead structure on which speed and lane control signs are displayed.

"The project has been the subject of a number of scope changes and associated cost increases, hence the difference in cost between the original approved cost and the final cost," NZTA said.

The agency did not say what type of asphalt it intended to use before deciding on the premium porous product, or what the additional road surface work involved.

The motorway opened to northbound traffic in June 2016 and to southbound traffic the following month.

Drivers were promised reduced congestion and shorter and more reliable journey times, but agency data last year revealed peak-time northbound journeys between Hobson St and Petone had actually become longer, while southbound journeys had only improved by an average of about 30 seconds.

The journey times were also blamed on poor driving habits, especially at the merge point with SH2 at the bottom of the Ngauranga Gorge.

Benefit cost ratios had not been reviewed since the motorway opened, but overall travel time reliability was the same as three years ago despite more traffic on the road, the agency said.

​Northbound travel times from the CBD to Johnsonville on SH1 had improved.​