The cost to implement a rail network to move freight from Northport through to the south of Auckland has been estimated at more than $2.5 billion.

A representative from the state-owned enterprise KiwiRail outlined the costs at the political rail forum in Whangarei on Monday night.

The forum was organised by Grow Northland Rail and attended by about 150 people.

KiwiRail's Dave Gordon took an apolitical stance as he broke the cost down into three sections.

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He said to get the North Auckland line up to scratch, tunnels, bridges, passing loops, a stretch of rail and the radio systems would need to be upgraded or replaced.

He said that would cost about $100 million. He said legacy works would cost another $200m over 15 years.

Mr Gordon said the link to Northport would be about $200m.

"If the rail to the North and from the North is such that you need to get a lot of traffic through Auckland then there are some very substantial investments to be made in the Auckland rail area."

He said KiwiRail had not done any estimates for that, but a third track would be required and that would cost about $2b-$3b.

Winston Peters has campaigned heavily on making Northport the number one port in the country and the benefits it would bring to Northland. He described the alternative options for moving Auckland port as "preposterous".

"We'll save billions and billions and billions by going to the only deep water port there is with the transport utility which we can build for a fraction of what they contemplate."

Of the Firth of Thames option he said: "You're going to have to virtually dredge half the Sahara to even get there."

Green Party's Julie Ann Genter said the party was committed to extending electrification in the North Island, extending the rail line to Marsden Pt and improving the North Auckland Line.

Channelling his inner debate moderator, Mr Peters interrupted her with: "What these people want to know, are you going to give us a modern, efficient, railway line from Hamilton, all the way to Northport and revive this province. Yes or no?"

"Yes." She answered, to a round of applause from the crowd.

Labour's David Parker said one of Labour's policies is to allow rail to compete into the Land Transport Fund.

He said the party's pledge was to make a prompt decision after having a port strategy that involved looking at Tauranga, Northport and Auckland.

"It may well be that Winston's right in his analysis of the cost of moving the port."

Shane Reti was steadfast throughout the night in the position that there had to be a business case for the option to be explored.

"If there are commercial quantities of freight at commercial rates, KiwiRail will carry that freight."

Earlier in the evening, he said that the local economy was growing, with 1500 people moving to Whangarei every year.

"Every new household in Whangarei injects $1000 of household expenditure into our local economy."

The forum consisted of three debates. The first was between Whangarei candidates Chris Leitch, Shane Jones, Shane Reti, Tony Savage and Ash Holwell.

The second and third debates were between the main political parties - NZ First, Greens, Labour and National. Firstly the parties debated a number of key election issues, then strictly rail.