Transport agencies, primary producers, local politicians and a wide range of businesses are among trainspotters waiting to see the shape the Auckland to Northland railway line is in.

An inspection is under way on railway tunnels, bridges and other structures to assess what is needed to bring the line up to modern standards.

The investigation is ''support'' work for a Marsden Point rail spur to Northport, said KiwiRail's group general manager investment, planning and risk, David Gordon.

"We are carrying out an assessment of the line to get more detailed information about what is needed to support a Marsden Point rail link, and enable faster trains carrying larger export containers, forestry and tourism services,'' Gordon said.

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The information will help KiwiRail determine time lines and costs for building the rail spur.

"The Government has indicated its strong support for the benefits rail brings for New Zealand by taking trucks off the road, improving road safety and reducing carbon emissions,'' Gordon said.

The Ministry of Transport is currently developing a business case for Northland Rail, KiwiRail said.

In October, KiwiRail began geotechnical investigations along some of the proposed rail link from Oakleigh to Northport. KiwiRail acting chief executive Todd Moyle also said then that scoping work would inform the business case being developed for the link.

But the chairman of an advisory group on upper North Island transport and port infrastructure, businessman and former Far North mayor Wayne Brown, said a business case was not necessary — the rail spur was already a done deal.

Brown said the first recommendation made by the Upper North Island Supply Chain Study working party set up earlier this year was for the spur to go ahead.

The case was presented to Regional Development Minister Shane Jones, Finance Minister Grant Robertson and Transport Minister Phil Twyford.

''We have been told that is going to happen, we've been told geotech work is under way across that corridor. We're waiting to see if there'll be one track or two, we don't need a business case,'' Brown said.

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KiwiRail currently runs only one weekday return service on the line to Auckland, predominantly carrying dairy and forestry.

"Much of the infrastructure on Northland's rail lines is very old, and while it is safe to run freight trains there are limits to what we can carry and how fast our trains can go,'' Gordon said.

"There are 13 tunnels on the line and as most were built more than 100 years ago, they are not fit for purpose for modern container freight.

''We've been inspecting these, and in some instances carrying out geotechnical investigations using drilling rigs to determine how we would widen and strengthen them.''

Geotechnical assessments have been carried out on a section of the route at Mata Hill, with further investigations scheduled for early next year near Oakleigh. The drills bore up to 30 metres into the ground for samples.