Ardern pledged the $20m to establish the first stage of the passenger service proposal - estimated to cost $10m.
If demand is there, Labour would look to invest in stages two and three of the plan, delivering services travelling up to 160km/h throughout the wider region.
The additional $10m would be invested over five years for operating costs.
When asked by the Bay of Plenty Times if she had spoken with the Port about whether the plan was possible, she said: "Not specifically, but I don't imagine it would have any impact on the work that they do."
Port of Tauranga chief executive Mark Cairns disagreed.
There are 78 freight trains a week on the rail line to the Port and this was expected to rise to more than 90 per week over the next 12 months.
"I'm surprised nobody has talked to us about it," Cairns said.
Labour's Transport Spokesman Michael Wood said the eastern main trunk line -
"which will be the bit of track that the Port is referring to" - had capacity for four trains per hour.
Wood said Labour was looking at just two trains a day using the line, "so it's got capacity of four per hour, there is absolutely capacity within there to deliver that".
"And no one user ... certainly, has a right to exclude others," Wood said.
"As the policy gets developed further and implemented, we'd certainly sit down and talk with the Port, but when you're developing a policy like this, you don't necessarily sit down with what is effectively a private company that currently has exclusive use of that track and has interests of potentially keeping others out."
Tauranga Mayor Greg Brownless said he did not think the rapid rail proposal would have an impact on the Port.
"We're only talking about a train or two a day, so I don't think that would interfere with the Port, no."
Kiwirail were not able to respond to requests for current line use by freight trains in and out of the port before publication last night.
Transport Minister Simon Bridges, who is also Tauranga's MP, said Labour's adoption of the passenger service was unrealistic.
"The Auckland-Hamilton-Tauranga rail line is our busiest freight route and simply doesn't have the capacity to also be a commuter rail line."
Tauranga Labour candidate Jan Tinetti said passenger rail was long overdue for Tauranga.
"The growth that we've had here has been phenomenal in the last few years and we need to actually look at public transport."
Labour also plans to:
• Boost transport investment in regional projects across the country by doubling the funding range in the Government Policy Statement.
• This will lift funding available for regional projects from $70m-$140m a year to $140-$280m a year.
• The increased funding will be available for all regions and for all transport modes.
• Ardern also set out a promise for her first 100 Days in Government - holding a roading summit in Wellington for the county's local bodies and transport bodies.