Chinese traders are halting purchases of Australian coal and coking coal as clearing times through China’s customs have at least doubled to 40 days or more, four traders at major buyers say.

The traders told Reuters on Monday only cargoes from Australia, the biggest supplier of the fuel to the world’s top consumer, were affected.

‘We have stopped ordering coal from Australia because it is unknown how long the restriction will last,’ said a manager at a Shanghai-based trading company who usually buys around 400,000 tonnes of Australian coal every month.

Customs clearance typically takes five to 20 days. Now it can be as much as 45 days, said the manager, asking not to be identified.

It was not clear why China had stepped up checks on Australian imports but tension between Beijing and Canberra has grown in recent months over issues of cyber security and China’s influence in Pacific island nations.

China’s General Administration of Customs did not respond to a request for comment. There was no immediate response from the Foreign Ministry to a faxed request for comment.

Authorities at some ports notified importers verbally this month that Australian thermal coal and coking coal would take longer than usual to clear, the Shanghai-based manager said.

A Beijing-based coal trader and a purchasing manager at a coke plant received similar notifications. None of the buyers were aware of the reason.

China has curbed coal imports in the past to support domestic coalminers and reduce consumption to tackle air pollution. It restricted Indonesian coal imports in 2017, citing high impurities and low energy efficiency.

The Beijing trader said the restrictions on Australian imports were ‘the first time for Beijing to curb coal imports from a specific country but without a reason’.

Reuters reported in January that dozens of ships carrying coal and iron ore were waiting to unload outside ports due to possible customs delays.

The most-active thermal coal futures on China’s Zhengzhou Commodity Exchange hit 594 yuan ($A123) a tonne on Monday, just shy of the three-month peak of 595.6 yuan, although the contract closed down 1 per cent 583.8 yuan a tonne.

Benchmark Newcastle high energy thermal coal prices have dropped to $88 a tonne, the lowest level in 19 months, consultancy Wood Mackenzie said.

The Shanghai trader said he had stopped buying from Australia and would purchase more from Indonesia and Russia.

The Beijing trader said she would reduce Australian imports.

A coal broker at state-backed mining group Minmetals said he had asked clients to put Australian imports on hold.

Refinitiv ship tracking data showed coal shipments departing from Australia’s Newcastle port to China fell 30 per cent last month compared with December to 18.19 million tonnes.

As of Monday, shipments scheduled to leave in February were 12.78 million tonnes.