New Zealand First leader and Northland byelection candidate Winston Peters is promising to upgrade the rail line to Auckland and build a rail link to the deepwater port at Marsden Pt to get full value from the port.

Mr Peters was in Whangarei on Monday ramping up his campaign after polls put him ahead in the race for the Northland seat left vacant after National MP Mike Sabin resigned.

And while Whangarei is not in the Northland electorate - so few, if any, of the 60 people who heard him speak in Cameron St Mall will be able to vote for him - he said Whangarei was the place to announce his policy for one of the region's major assets, the port.

He took time to slate National's announcement of replacing 10 one-lane bridges in the Northland electorate as a bribe worth between $32 million-$69 million.

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"These are promises that National has made countless times before. They were in the Northland Regional Transport Plan in 2009. They were taken off when Steven Joyce shrank rural road funding to meet his pet Roads of National Significance plan."

Mr Peters topped a 3News/Reid Research poll last week that gave him 35 per cent support in the race, compared with National's Mark Osborne on 30 per cent and Labour's Willow-Jean Prime on 16 per cent.

On Monday, Mr Peters took the chance to make his own byelection promises, including upgrading the Auckland-to-Northland railway line, to stop the plans for the two large wharf extensions by Ports of Auckland and bring further container port development to the deep water port at Marsden Pt. He said the deep water port was the national place to develop a container port, with plenty of available land. "And we will upgrade the Auckland-to-Northland railway line and build the rail link to your port."

He said if he won he would have the numbers in Parliament to achieve the promises.