Port Waratah Coal Services (PWCS) employees will conduct protected four-hour stoppages, from 11pm to 3am, every night from Thursday until next Wednesday.

The Maritime Union of Australia says the stoppages will occur because a number of conditions haven't yet been agreed upon after more than nine months of negotiations.

"We are close to agreement," MUA national assistant secretary Ian Bray said.

"However, there remain a few unresolved issues that the workers consider important enough not to walk away from and, as a result, the workers are taking this protected industrial action."

He says the union hopes for a speedy resolution to the standoff so workers can "get on with the business of shipping coal to PWCS clients".

A PWCS spokesman says the company is disappointed by the latest action but the stoppages won't halt operations over the next week.

"We've got contingency plans in place and we'll be unloading trains and loading ships according to that plan safely using trained staff," he told AAP.

He said negotiations had been carried out in a "good spirit", adding PWCS was hopeful of finally reaching an agreement.

"We're not far distant in resolving the issues that are in contention."

Mining giant Rio Tinto, through its subsidiary Coal and Allied, is a 30 per cent shareholder in PWCS.