The Australian dollar took a hit on Friday after China announced an indefinite ban on coal imports.

Customs officers at the key northern port of Dalian have reportedly stopped Australian coal imports ahead of a new quota regime that will cap imports at 12 million tonnes a year.

It appears that only Australian coal is being targeted.

By 6:00pm (AEST) on Thursday, the Australian dollar had tumbled below 71 US cents.

The Dalian customs zone covers five harbours — Dalian, Bayuquan, Panjin, Dandong and Beiliang — which serve the heavily industrialised steel-making northern part of China.

The ban is currently limited to coking coal (used in steel making), but there is strong speculation that it will spread to other types of coal and ports.

Australian exporters had seen increasing delays trying to get their goods through the Dalian ports in the lead up to the ban.

They say the same delays are now occurring at other Chinese ports, which handle both coking and thermal coal.

Coal is Australia's most valuable export, bringing in about $64 billion a year, a quarter of which is from China.

CBA commodities analyst Vivek Dhar says Chinese officials are sending a message.

“It has crystallised the fear that something is going on, that China is targeting Australian coal,” he told the ABC.

Trade Minister Simon Birmingham has asked Australia's ambassador in Beijing to seek clarification of the reported ban.

Any displaced Australian coal shipments are expected to find a market, but most likely at a discounted price.