The government announced yesterday it would set up a new Crown-owned company to buy two new ferries, to begin operation in 2029.
However, details about the cost of the new ferries and whether they would be rail-enabled were not revealed.
Finance Minister Nicola Willis said more details would be revealed in March, as the deal was still "commercially sensitive".
It comes a year after the government decided to cancel a previous replacement plan, called iRex, because the two mega ferries needed major terminal and port upgrades at Wellington and Picton, which would have pushed the overall cost to $3 billion.
Rail and Maritime Transport Union general secretary Todd Valster said Ms Willis’ announcements— and the implications of them — were "vague".
"We think the large rail-enabled ferries would have been the way to go, even if they had to downsize the infrastructure in Wellington and Picton from what it was going to be, because it obviously got out of control," Mr Valster said.
"I just think that this government has made a very poor decision today, but then they haven’t really made a decision apart from saying that they’re going to be able to tell us in March."
Shortly after the announcement, Taieri MP Ingrid Leary issued a statement saying the fact no commitment had been made for rail-capable ferries was "setting up Dunedin’s iconic Hillside workshops to fail, putting at risk hundreds of jobs even before the ribbon is cut".
"Without the link across the Cook Strait provided by rail-enabled ferries, the whole South Island’s rail network is at risk, and that will be the death knell for the wagon assembly market.
"It will effectively turn New Zealand into two separate railways for the purposes of freight movement and rail maintenance," Ms Leary said.
Mr Valster said it was far too early to say whether yesterday’s announcement would affect Hillside.
"Severing the railways is the bad news. At this stage, it would be far too early for me to say whether Hillside has got a bleak future."
The appointment of Deputy Prime Minister Winston Peters as minister for rail was potentially positive news, Mr Valster said.
Mr Peters had previously spoken positively about Hillside, he said.
"But what [Ms Willis] needs to stop is calling the Korean ferries Ferraris, because — if they do actually know their cars — Ferraris are horrendously expensive, and those ferries that were ordered from South Korea were not horrendously expensive."
KiwiRail chief executive Peter Reidy said it would work with all parties to support the procurement and introduction of the new ferries when they were ready to enter service.
"We note that rail-enabled ships are still an option. However, if the new ships are not rail-enabled, we will be able to continue to efficiently move rail freight across Cook Strait in the same way we do now when our only rail-enabled ship, Aratere, is unavailable."
