The state pension fund could be used to open up Auckland's harbour bridge to pedestrians and cyclists.

That follows a conditional funding offer to a trust planning to build a $28 million tolled pathway across the bridge, which has always been off-limits to non-motorised traffic.

The offer comes from the Public Infrastructure Partnership (PIP) Fund, which is managed by Wellington-based Morrison and Co and has the NZ Super Fund as its largest investor.

But it still remains subject to Auckland Transport agreeing to provide financial contributions if the project falls more than 25 per cent short of revenue forecasts during the first four years.

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SkyPath project director and veteran bridge crossing campaigner Bevan Woodward does not believe that will happen but says that even under a worst-case scenario, no more than $4 million of public money would be needed over that time.

Auckland Council would in return gain ownership of the pathway after it is paid off by tolls over 20 years, and would receive a 75 per cent share in that time of any revenue surplus to what is forecast in the project's business plan.

Tolls of $1.95 to $6 are proposed for each crossing, depending on how long cyclists and pedestrians spend on the bridge.

Morrison and Co has in a letter to Mr Woodward described investing in the project as "an excellent fit" for the PIP Fund's portfolio of "social infrastructure" assets, which include an agreement with the Government to build, finance and maintain two new schools at Hobsonville.

Auckland Transport spokeswoman Sharon Hunter said an independent feasibility assessment had been commissioned from which a report was expected by the end of October.