A draft budget prepared by council staff for this week's council pre-draft budget meetings showed work to make the tunnel safe for public use could cost $1.689 million.
That was the estimated maximum cost of the four-stage project, based on the council's contractor rates.
However, trust member Gerard Hyland was confident fundraising by the trust, volunteer labour and other in-kind offers of support would significantly reduce the bill.
The draft budget was included in a council staff report prepared for councillors on the proposed southern cycleway, which would run from Mosgiel to Dunedin's city centre via the tunnel.
The first two stages of the project would include formalising the relationship between the council and the trust, and protecting electrical cables and a gas pipeline within the tunnel, at a total cost of $326,000.
The bulk of the costs, totalling $1.338 million, would come in stages three and four, and it would be left to the trust and any partners to raise the money, the budget shows.
Work in stage three would cost $828,000 and include protection of a council wastewater pipe running through the tunnel ($320,000) and the installation of atmospheric monitoring equipment ($400,000), if required.
Further "optional enhancements" could also come in stage four, including CCTV and lighting, a ramp at the Burnside end of the tunnel and a path from Sydney Park, together costing $510,000.
The report by council transportation planning manager Sarah Connolly recommended the project be included in the council's long-term plan, but made no recommendation on funding the bulk of the costs.
Her report noted there was money for the project in the council's budget, with $150,000 of $158,000 allocated for initial investigation work still unspent.
Ms Connolly told the ODT initial project management work had been conducted in-house, and a council working party looking at the project had negotiated significantly reduced rates for geotechnical and gas-monitoring work inside the tunnel.
Her report recommended the remaining $150,000 be used to help fund the early stages of the tunnel project.
Mr Hyland told the ODT the council's cost estimates were "surprisingly high", but he was confident the trust could complete the work.
"We would be doing the same work, or similar work, but in-kind, so we wouldn't get anywhere near that sort of cost level."
He hoped the tunnel's gates would be opened to volunteer workers and others to enter "at their own risk" by October, in time for the 140th anniversary of the tunnel's construction.
