The number of Chinese ports restricting or delaying Australian coal imports has continued to rise, threatening to end the export bonanza that is bloating federal coffers and signalling possible painful long-term structural change to the economy.

In an ominous development for Australia’s trade balance and federal budget, traders and buyers in China reported on Thursday that the hold-ups for Australian shipments that began in February have spread from the northern port of Dalian to Fuzhou in the south-eastern Fujian province and Rizhao in Qingdao.

Industry news agency Platts reported one market source as saying that Australian thermal coal used for power generation has been “largely forgotten” by Chinese buyers as they switched preference to delay-free imports from Indonesia and Russia.

The reports have seen a sharp drop to the share price of Australian pure play coalminers. New Hope Corporation shares were down more than 20% this week after it warned of a sharp fall in exports bound for China, while Yancoal has fallen 9%.

But the impact will not be limited to the coal sector. The commodity is Australia’s second biggest export earner and demand from China accounts for 3.7% of Australian GDP – although half of that comes from exports of coking coal, which is used for steel production.

The benchmark price for high-energy Australian thermal coal has enjoyed a buoyant couple of years of prices above $100 a tonne. The figure has now dropped below $US90 a tonne for the first time in almost two years, threatening some of the export-duty windfall Josh Frydenberg can expect to highlight in his budget next month. A steep fall could also hurt the Australian dollar.

It remains unclear why Australian imports have been targeted for the hold-ups at customs. Official explanations from China have mentioned the need for environmental checks and even a suggestion that imports needed to be checked for radioactivity.

However, few doubt that there is a considerable political dimension. Tension between Canberra and Beijing has been growing in recent years amid concern about China’s military ambitions in the South China Sea but has intensified over the reach of technology companies such as Huawei and ZTE, the “state actor” cyber-attack on Parliament House and the rejection of Huang Xiangmo’s visa.

However, there are also intriguing market-centred reasons why China might want to reduce its reliance on foreign resources and which suggest that Australia’s coal industry could be facing a more existential threat.

Alex Turnbull, of the Singapore-based hedge fund Keshik Capital, said the port slowdown was the result of wide-ranging changes in the Chinese economy. A huge expansion of rail infrastructure in the past decade had enabled China to begin exploiting its vast inland coalfields in areas such as inner Mongolia and Shanxi in a way that was not previously possible, he said.

Thermal coal imports were unheard of before 2008, he said, but they boomed to fuel the massive economic stimulus ordered by Beijing in the wake of the global financial crisis. At the same time, the country’s national development commission started adding millions of tonnes in freight capacity to the Chinese rail system so that cheaper domestic coal could be shipped to coastal centres of heavy industry and population.

“What is happening now is that the infrastructure schemes have caught up with demand and therefore imports are going to dry up,” Turnbull said. “We’ve been waiting for the time when China turns off the demand for Australian coal imports and this appears to be the moment.”

He conceded that there was a “political element” to what was happening but emphasised the seismic shift occurring below the surface. “Focusing only on those political issues risks missing the bigger overall picture which is that the market for thermal coal is going to shrink. The total of Australian thermal coal imports to China is going to go down,” he said, adding that China also wanted to boost its current account surplus by reducing imports and supporting its currency.

Other analysts see a less decisive change and say the port slowdowns are a blunt and deliberate move by China to bring about a correction to coal prices, which remain above what the Chinese government expects to pay.

“At a fundamental level, this is happening because the Chinese government wants to protect its own coal industry,” Rory Simington, principal analyst at global energy consultancy Wood Mackenzie, said. “The government wants to have prices high enough for its own producers. But it wants to keep imports out so that the benefits flow to domestic producers and not to Australia and elsewhere.”

Import slowdowns of the type affecting Australian coal now were not new, he said, but the new element was that Australian coal was being specifically targeted. The proof was that while Australian coal prices have fallen in the wake of the restrictions, the price of Indonesian coal has risen.

“People who are buying Australian coal have switched,” he said. “If you have a vessel that is sitting outside a port for three months waiting for clearance you’re going to buy Indonesian or Russian coal which isn’t going to be delayed.”

Although Simington does not believe that Australian coal exporters face a mortal threat, producers will have to divert their coal to other markets if Chinese demand dries up completely. Alternative buyers are principally India, Europe and other south-east Asian countries. But prices will be lower, he said. A fall of $5 per tonne in the cost of low-grade, or high-ash coal, which makes up the vast majority of exports to China, could cost Australia $200m a year, he said.

With no official pronouncements from Beijing about its intentions for Australian coal, Andrew O’Neil, professor of political science at Griffith university, said the situation was difficult to read and had left many analysts puzzled as to China’s motivation.

But he argued that Beijing’s diplomacy could be more nuanced than many supposed and that the leadership had a track record of using subtle signalling to manage its relations with other countries. The coal import ban could therefore be seen as “very carefully calibrated”.

“This is not going to torpedo the Australian coal industry, but China could torpedo Australia’s coal industry if it wanted to ... so they’re looking to send a signal,” he said. “If I had to put money on it I would say there was a political and strategic dimension to this decision.”