The strike, proposed by the Rail, Tram and Bus Union against the Asciano subsidiary rail group, was already extended to 48 hours from tomorrow after federal Industrial Relations Minister Bill Shorten failed to break the impasse.

The action will prevent at least 300,000 tonnes of coal from reaching the Port of Newcastle, with the loss of $25 million of exports a day to the NSW economy, according to Pacific National.

"Two days itself is a nuisance more than a long-lasting problem," coal economist Donald Barnett reportedly told the ABC.

"But this of course depends on the weather and if we get a nice downpour then all these things really exacerbate the situation.

"So it's not a major problem now but if luck goes against you, it could be the start of a major problem."

Barnett said the perception of Australia as an unreliable supplier of thermal coal because of industrial disputes could tarnish its reputation and make it easier for competing exporters, such as Indonesia, to make further inroads into the Australian coal export market.

"We have a major producer who produces more than us on our doorstep, which is Indonesia," he said.

"This makes it easier for the Indonesians to sell their coal and more difficult for us but in the short run, if it's only two days then it's no major problem."

The proposed protected industrial action by RTBU members had the potential to cause “significant harm” to Whitehaven, managing director Tony Haggarty told ILN.

“Accordingly, Whitehaven has requested in writing that that RTBU withdraw the notice of protected industrial action for 8 February 2013 and provide an undertaking that it will not give any further notice of protected industrial action or engage in protected industrial action in respect of the bargaining with Pacific National,” he said.