Finn Blackwell

The grounding of the only rail ferry in the Cook Strait demonstrates how fragile the inter-island connection is, Transporting New Zealand says.

The Interislander ferry Aratere had been on a freight-only sailing on Friday when it suffered a steering failure and ran aground about 10pm.

It was refloated on Saturday night and will to safe anchorage in Picton Harbour Sunday.

Transporting NZ interim chief executive Dom Kalasih said the organisation needed assurances from the government that something would be done to provide a reliable service.

"The incident has really just demonstrated how fragile that link is," he said.

"We'll certainly be advocating hard for greater certainty from government as to what the plan is to make that link more reliable."

Kalasih said there should not be significant long-term impacts to freight networks because of the Aratere running aground.

KiwiRail chief executive Peter Reidy ways freight is being carried on the Kaitaki, pictured, while Aratere is out of action. Photo: Supplied / KiwiRail

KiwiRail chief executive Peter Reidy said any delays to the freight network would depend on how long the Aratere was out of service.

"We're working right now with our customers of the rail freight, getting it onto (Aratere's sister ship) the Kaitaki, to be able to move it across the Cook Strait, which we do regularly," he said.

"When Aratere is in a wet-dock or a dry-dock, we'll move it through the other ships."

In a statement, Reidy acknowledged the disruption the grounding had caused.

"We know this incident is extremely disruptive for our customers and appreciate their patience as we work with them to organise alternative arrangements," he said.

But Maritime Union spokesperson Victor Billot said there could be significant hold-ups to freight with Aratere out of service, as Kaitaki was not rail-enabled.

"Any freight going across the Cook Strait, if it's rail-based, it has to get put on trucks, then taken onto the ferries, then taken off the trucks, then put back on the rail," he said.

"That's not an ideal situation in terms of double handling, and emissions and so forth, and the time it takes, and the cost."

KiwiRail said it could not give a timeframe for the Aratere's return to service.

It said the ferry would not operate until it had been released by Maritime New Zealand, which was investigating the grounding.

This story was originally published by RNZ