The Interislander ferry Aratere is grounded in a bay near Picton.

The ferry had been on a freight-only sailing when it suffered a steering failure and crashed about 9.45pm on Friday. No one was injured and there was no environmental damage, KiwiRail said.

The plan to refloat the vessel has been pushed out but authorities are helping crew and passengers off the ferry.

The grounding of New Zealand's only rail ferry is going to cause "issues" for inter-island freight, a union representative says.

The Aratere ran aground just out of Picton around 10pm Friday with eight drivers and 39 crew on board.

No one was injured, but the vessel could not be moved until high tide after 8am on Saturday, Interislander general manager Duncan Roy said.

Maritime Union communications officer Victor Billot told RNZ the Aratere was New Zealand's only rail-enabled ferry, meaning it could be used to transport trains across Cook Strait.

In that way it was an extension of the Main Trunk Line and formed an integral part of the rail network.

The ferry's grounding meant there would be "issues" with getting freight between the North and South islands, Billot said.

"There are going to be issues there. I have no idea how that's going to play out."

The incident showed "just how important those ferries are for the connection between the islands", he said.

The Aratere, along with its sister ferries Kaiarahi and Kaitaki, carried billions of dollars of freight every year, he said.

"We're obviously going to see over the weeks ahead what the impacts are going to be of this incident."

Roy did not answer questions about impacts on freight when asked by RNZ shortly after the ferry ran aground, saying it was too early to address that issue.

"Right now I'm totally concentrated on our passengers on board and our crew on board - that is our main focus.

"Right now we're totally focussed on recovering our vessel but most importantly making sure our people are safe and sound."

Roy said the grounding came after a reported steering failure, but would not go into further details, saying he did not want to pre-empt an investigation that was under way.

There was no word on the severity of the issue or how long it would take to fix.

In 2013, the Aratere lost a propeller during a voyage, leading to the ferry being out of action for six months while it underwent repairs.

KiwiRail hired a ship from Poland to cover the route during that time. All up, the repairs, ship hire and lost revenue cost KiwiRail between $20 million and $30 million, its then-chief executive Jim Quinn said at the time.