The Boggabri Coal workers claim they get about $40,000 a year less than at Idemitsu mines in Muswellbrook and Ensham "with no explanation".

It's unfair not only to them and their families but also local economies, union branch secretary Dave Boxsell said.

Higher pay would "absolutely" make a difference to the workers' hometowns in north-west NSW suffering the effects of drought.

"We want to be able to support the areas we all live in," Mr Boxsell said.

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Union district vice-president Jeff Drayton said: "The economy survives off disposable income".

"That's what businesses work on ... whatever extra income you've got, people will spend it."

The workers have been on a 48-hour strike, expected to end tomorrow, as they try "to win back conditions" after their employer changed from contractor Downer to the mine owner, Idemitsu.

An estimated 300-plus of them gathered for a rally in Gunnedah yesterday, to hear updates on the union's latest negotiations with the company.

There are "12 outstanding issues" in the enterprise bargaining, including the overall pay packet, redundancy pay, long service leave rules and evidence required for sick days.

Union district vice-president Jeff Drayton said there was some urgency to resolving them because the previous agreement was already "expired for a few months" and a new one would take months to finalise.

He said the company had increased its pay rise offer from a 9pc to 11.5pc over four years but didn't want to backdate it.

"It's been taking around about five months or six months for that approval process to be finalised in the Fair Work Commission ... the 11.5pc effectively, you get over a five-year period, not a four-year period, because it doesn't kick in [until] we get the agreement done.

"Our position is that the pay rises kick in and get backdated to when this agreement expired."

Idemitsu responded to the latest industrial action by saying it had "continued dialogue and negotiations in order to finalise an agreement that is in the interests of our workers".

"The proposed new agreement we have discussed with workplace representatives, contains higher salaries, significantly increased benefits and a greater say for employees in the rosters that they work," a spokesperson said.

"The remuneration package proposed would be the highest in the Gunnedah region, with far better terms and conditions whilst maintaining the current lifestyle roster.

"Boggabri Coal has a proud history of supporting the local community through direct employment, significant spend on local businesses and direct sponsorship of local community groups.

"The revised agreement we have put on the table will provide stability and security for our employees, their families and the local communities of Boggabri, Gunnedah and Narrabri."