The strikes began last Thursday and would continue through to this Thursday, Reuters reported.

The strikes are being coordinated by the single bargaining unit representing workers from the Maritime Union of Australia, the Transport Workers Union, the Electrical Trade Union and the Australian Manufacturing Workers Union.

Port Waratah Coal Services has denied that the strikes have cut export volumes, although it said they have slowed loading at one terminal.

"If there's any deterioration in (exports) it's basically due to (lower) demand," PWCS spokesman Paul Chamberlin told Reuters, acknowledging that the company was likely to fall short of its 110 million tonne target for exports in 2013.

But MUA Newcastle branch secretary Glen Williams told Platts that the strike action, which includes multiple strikes since mid-May totalling more than 100 hours, meant the shipping company had not met its terminal target for June.

He said PWCS' Newcastle terminals had a ship loading target for June of 9.67Mt and the actual volume of coal exports shipped for last month was 9.24Mt.

About 220 union workers are involved in the strikes, which the unions say are because of the perceived unwillingness of PWCS to reach agreement on the major outstanding enterprise agreement issues.

The union will meet with PWCS again next Tuesday.