Canterbury's manufacturers are frustrated that the on-again, off-again Lyttleton port strike is causing delays in receiving crucial materials and pushing freight costs up.
Lyttleton handles more than half of the South Island's container volume, including 70 per cent of imports.
It is the country's largest coal facility and handles a significant quantity of agricultural exports.
Port company management and the Rail and Maritime Transport Union have been unable to resolve disputes around pay and weekend rosters, and the union has called and suspended strike action twice since last week.
Christchurch-based Manufacturers' Network chief executive Dieter Adam says he sent out an email on Tuesday evening asking his members about the impact of the industrial action, and got a quick and sometimes angry response.
"This is affecting manufacturers who are trying to get their products out of the port to meet delivery deadlines, and those who are waiting for input goods to arrive," he said.
"Some have reported the lack of input goods is expected to severely impact their manufacturing businesses, potentially delaying their own production."
Some were moving goods through the Timaru port, at a significantly increased cost - between $1000 and $4000.
Although members of the Rail and Maritime Transport Union haven't actually spent much time out on strike, on-off threats of industrial action mean Lyttleton Port Company has been forced to divert ships to other ports, some going as far as Auckland or Tauranga.
Strike notices are still potentially in place until March 25, although the union has suspended its strike action from midnight until Tuesday next week.
This meant the port could start bringing ships back into Lyttleton, says port operations manager Paul Monk.
Mr Adam said it was annoying that the two sides appear to be close to an agreement, yet haven't got a deal.
"South Island freight services are being held to ransom over what currently appears to be a relatively small amount of money which is still in dispute.
"This is particularly frustrating when many of the workers involved may already be on significantly higher wages compared to other workers in the transport industry, and stand to receive further pay increases from the offer put on the table by the port."