Supermarkets have been stocking up and South Island construction companies are bracing for even more shortages with two Interislander ferries out from today.
The Aratere leaves for Sydney today to undergo scheduled maintenance until 2 October, but the Kaiarahi is already out of action due to problems with its gearbox.
It was expected to be out for some weeks, which leaves only one KiwiRail ferry - the Kaitaki - in operation for two weeks.
Transporting New Zealand chief executive Nick Leggett said the disruption could not have come at a worse time.
"It's not really good enough in a time when we've got an economy that badly needs its supply chain to function to have this occur.
"Whether it's poor planning - it's certainly not underfunding because they get plenty of money - it's not acceptable for the key carrier across the Strait to allow this capacity to be out at such a crucial time."
It put trucking companies, and the contracts they needed to fulfil, in a precarious position, he said.
"They've been let down by a key part of the chain and goods are not gong to be able to be delivered and trucks are not able to run and they will have to play catch up when the ferry capacity improves again."
Mainfreight managing director Don Braid said as much notice as possible had been given and drivers had been working to move more stock as customers upped demand ahead of the capacity drop.
"We've moved quite a bit of extra freight over these last eight or nine days in preparation, so we're hopeful we'll get through okay."
All the freight would be moved, it would just take longer, he said.
"We're sourcing additional inter-island space that's available on coastal shipping as well as on Bluebridge so we won't leave freight behind but our advice to our customers is not to be thinking 'just in time', but pre-planning and allowing a longer transit time for the freight to get to the South Island."
Countdown had been shipping more than it usually would between the two islands in order to make sure all stores were well-stocked during this period.
Foodstuffs had also worked to secure supply and both outlets asked the public to shop as they normally would.
And while KiwiRail said freight would be prioritised during the single-vessel sailings, the news was just another blow in a series for builders in the South Island.
Canterbury builder Carl Taylor, from the building co-operative CBS, said it was terrible timing.
"We're already aware that a lot of the timber, particularly the pre-cut is out to March now and that's bad enough as it is so if the ferries are out for two more weeks it's just going to add to a struggling industry that already can't keep up."
The re-starting of some manufacturing in Auckland was good news, but he said it was no good if it could not get down the country.
"I've been in the game for coming up 21 years now and I've never seen delays this bad across the board, ever.
"Just cannot get gear."
The other inter-island ferry Bluebridge did not return RNZ's request for comment on what its role was while the two KiwiRail ships were out of action, or if it would be adding more sailings.
The Aratere was doing extra sailings to shift as much as possible before its departure today.