Lyttelton Port workers are heading to Christchurch City Council on Thursday to protest what their union says is the port company's threat of a potentially illegal lockout.

The port and Rail and Maritime Transport Union were in mediation on Wednesday to try to resolve a dispute over late-night rosters - they didn't come to an agreement, though the union did call off planned strike action ahead of more talks on Monday.

RMTU organiser John Kerr said Lyttelton Port is refusing to pay some of the RMTU workers for their scheduled shifts this weekend, saying it has diverted ships away from Lyttelton because of the strike threat, and now there isn't any work for them to do.

Around 70 cargo handlers are being refused pay, according to the union.

"In our view this amounts to an illegal lockout - we gave (Lyttelton Port) two weeks' notice of our industrial action, as required by law, they have given us none," Mr Kerr said.

"What's so disappointing is that the talks yesterday brought us within a whisker of a reaching a settlement. We were confident that with a bit more effort we could reach a deal."

The union is appealing to the city's mayors and councillors to step in, as Lyttelton Port is wholly owned by Christchurch City Council.

"Refusing to pay workers who have called off strike action makes no sense, it simply inflames the situation," Mr Kerr said.