About 100 Construction Forestry Mining and Energy Union (CFMEU) workers are striking to protest against the Port Corporation's proposal to reduce the number of employees covered by the union agreement, Bob Timbs, a union spokesman, said.
"The strike action is not about the remuneration," Bob Timbs, a union spokesman, said.
The union is asking for a 4.5 percent pay raise and the port has offered a 4.3 percent increase.
The Port Kembla Coal Corporation declined to comment.
The strike at the small port in New South Wales state may provide some support to weak metallurgical coal prices, but prices are more likely to get a boost if strike action resumes at BHP Billiton-Mistubishi Alliance's (BMA) seven mines in Queensland state, according to analysts.
Spot metallurgical coal prices have tumbled to around $220 per tonne, down from the 2011 high of $350 per tonne, when prices were pushed up by flooding that brought coal production to a near standstill in Queensland state.
About 3,500 unionised workers at the BMA mines have been staging rolling work stoppages since they first approved strike action in June 2011. The total workforce at the mines is around 10,000.
CFMEU spokesman Stephen Smythe said the union was not satisfied with BMA's current proposed contract and was convening union members to discuss the agreement on Thursday and Friday.
BMA-operated mines have a combined output capacity of more than 58 million tonnes per year of mostly metallurgical coal, representing about a fifth of annual global trade.
Analysts have estimated that a full week of 12-hour stoppages at the mines would cut production by up to 1 million tonnes.
BHP Billiton-Mistubishi Alliance (BMA) mines have faced a series of work stoppages since the CFMEU approved strike action in June in an effort to get greater job security and more pay for its members as rising commodity prices boost mining sector profits.