Whitehaven, which operates seven NSW mines led by the Maules Creek and Gunnedah open-cuts and the Narrabri underground mine, issued a quarterly report this week that largely exceeded or met the guidance it had given to the stock market.

Its share price rose from $3.65 to $3.83 on Thursday as a result, but it fell back yesterday to $3.78 in late afternoon trade.

Whitehaven said that during the 2018-19 financial year, 81 per cent of its coal was thermal coal for power stations with the remaining 19 per cent coking coal for steel making.

Coal trades globally in $US and Whitehaven says it achieved an average price of $US100 a tonne for thermal coal and $US119 a tonne for coking coal.

At the exchange rates prevailing this year, that converts into likely prices of more than $140 a tonne for thermal coal and $167 for metallurgical coal.

Historically, the Australian dollar has tended to move broadly in line with coal prices, meaning that big increases trading prices are often cancelled out when converted into a strong $A.

But industry analysts are wondering whether the historical nexus has broken, with the $A falling by 10 US cents in 2018, and remaining around the same 70 cents value ever since, helping the coal industry and other Australian exporters.

On the cost side of the equation, Whitehaven had forecast a per-tonne cost of $67, with the final figure to be released with its final year results due next month.

The Hunter's biggest coal company, Yancoal, has also benefited from the low exchange rate, telling the market earlier this year that it expected average production costs of $62 a tonne, meaning its mines were operating with "industry-leading cash margins". Yancoal's production grew dramatically when it bought the Coal & Allied mines - Hunter Valley Operations and Mount Thorley/Warkworth - from Rio Tinto.