Australian trade minister Simon Birmingham has said Australian coal being exported to China is being delayed at customs for up to 40 days as confusion reigns about the Asian nation's import policies.

Normally, coal deliveries would clear customs within one to two weeks, according to a shipping broker, but a backlog has been building after a report earlier this month suggested that some Chinese ports had blocked shipments of Australian coal.

An official at Dalian Port Group told Reuters that some of the country's biggest ports had banned imports coming from the Antipodean nation and will cap overall coal imports for 2019 through its harbours at 12 million tonnes.

The report caused the Australian dollar to briefly decline, but the report was swiftly denied by Mr Birmingham, who said coal shipments to China have been delayed before.

A Chinese official also denied any kind of ban and insisted the delays that have been noticed have come as a result of efforts being stepped up to check the quality of imported coal.

"In recent years, China Customs has found that some of the imported coal didn't meet our environmental protection standards when it carried out inspection and testing on safety and quality risks of the imported coal," explained Foreign Ministry spokesman Geng Shuang at a media briefing.

Mr Birmingham said in a statement that the Australian government "trusts that our free trade agreement commitments to each other will continue to be honoured".

There have been simmering trade tensions between China and Australia since the latter recently revoked the visa of a Chinese businessmen, but Beijing has also been attempting to restrict coal imports more generally as it tries to support domestic prices.

Customs data shows China bought 28.26 million tonnes of coking coal from Australia in 2018.