Auckland is about to gain its first rail network extension since 1930 as a $90 million branch line for Manukau's city centre moves into its main construction phase.

Transport Minister Steven Joyce, under fire from regional politicians for delaying Auckland's rail electrification project while fast-tracking highways, will today lead a sod-turning ceremony for the 2km mainly double-tracked link between Manukau and the North Island trunk line at Puhinui.

Government-owned KiwiRail is spending about $50 million on the link, which will include a north-facing junction with the main trunk and a 300m trench at its eastern end.

Manukau City is contributing almost $33 million for a rail and bus hub and earthworks.

The Auckland Regional Transport Authority will cover other costs of a new railway station to be built in the trench, 7m beneath Hayman Park and a proposed campus for up to 25,000 Manukau Institute of Technology students.

Manukau's financial share includes a $7 million regional grant from the former Infrastructure Auckland, and Transport Agency subsidies for two new local roads above the trench and a bridge over it.

The line will run parallel to the new motorway link being built between State Highways 1 and 20 for $210 million and will initially carry trains every 20 minutes or 30 minutes at peak times.

Manukau Mayor Len Brown says he expects the new station to rival Newmarket as the second busiest in the region behind Britomart when it opens in early 2011.

It will be ready to serve the first stage of the new campus when that opens to an inaugural batch of 1500 full-time students at the start of the 2012 academic year.

The link will be the first extension to Auckland's 100km urban rail network since the eastern line was built through Glen Innes in 1930, although the Onehunga branch line has had its tracks relaid, ready for a resumption of passenger trains next year.

Passengers arriving at the Manukau station will use an escalator to reach Davies Ave, where they will step out into a covered bus interchange.

That will form the main hub for most bus services in southern Auckland.

About 600,000 passengers a year are expected to use the railway station, and twice that number is forecast for the buses.

KiwiRail says the line will be included in the Auckland rail electrification project, due to be completed by 2013.

The regional transport authority has yet to confirm whether it will be able to run trains every 20 minutes or 30 minutes at peak times from when the line opens, but hopes to increase that to every 10 minutes once it is electrified.