Rail advocates have launched a petition calling on the government to ensure the new Cook Strait ferries can carry trains.
The Future is Rail group said it has deep concerns over the government's failure to announce a rail-enabled replacement after it cancelled the iRex ferry project last year.
The project was cancelled in December after the government declined KiwiRail's request for an additional $1.47 billion funding.
Two rail-enabled ferries were set to be delivered by 2026, but construction had not yet started, and a ministerial advisory group was tasked in February with coming up with an alternative plan.
Finance Minister Nicola Willis said this week that no discussions or proposals for replacement ferries had gone to Cabinet in the nine months since the coalition government cancelled the new fleet.
The government has been considering advice from an independent advisory group on replacement options for the Interislander ferries since June.
A petition launched on Tuesday calling on the government to ensure the replacement Cook Strait ferries were rail-enabled, had gathered more than 2000 signatures.
The Future is Rail spokesperson Dave Macpherson said the cancelled ferry project, combined with a lack of government investment in South Island strategic rail and other transport infrastructure, would have long-term negative impacts.
The group wants politicians to publicly advocate for the replacement of the existing Interislander fleet, with modern rail-enabled ferries to maintain the connectivity of the country's rail network.
"Without rail-enabled ferries, the two islands' rail networks will be disconnected, restricting the movement of goods and jeopardising the South Island's entire rail network.
"That means more trucks on our already battered roads."
A KiwiRail spokesperson said the organisation would continue to work with the government to find the best pathway for Interislander to continue providing a safe, reliable Cook Strait connection.
Of its three ferries, only Aratere is rail-capable. It normally offers four crossings daily - two from Wellington, and two from Picton.
Allowing for scheduled maintenance, it does around 1100 rail-capable sailings each year - moving approximately 24,000 rail wagons annually across Cook Strait.