The ongoing strike by BHP Billiton (ASX: BHP) miners is expected to reach a peak on Thursday with about 4,000 employees from six mines expected to walk off their jobs following a deadlock in enterprise bargaining discussion.

The coal unions earlier rejected a 15 per cent pay hike offer by BHP spread over three years, including a one-time $15,000 bonus after 80 per cent of union members who belong to the Construction, Forestry, Mining and Energy Union (CFMEU), the Communications, Electrical and Plumbing Union and the Australian Manufacturing Workers Union voted against the offer. .

CFMEU District President Stephen Smyth said the industrial action's aim is to bring back BHP to the negotiating table to tackle who will perform safety roles at the mine. Mr Smyth said salaries are not a major issue in their action.

The prolonged industrial action has dented Queensland's income by $60 million in lost royalties, while BHP has lost a larger amount in sales.

BMA Asset President Stephen Dumble lamented that all the solutions presented by the joint venture between BHP and Mitsubishi were thumbed down by the unions.

"The current BMA enterprise agreement negotiations is at an impasse over exactly this type of issue where unions want to ignore current market and employment trends and instead preserve a status quo position for their own industrial ends," Couriermail.com.au quoted Mr Dumble.

"Further industrial action is unnecessary and would be harmful for all concerned. As the record has shown, it will not help resolution of an agreement," AAP quoted BHP spokeswoman Kelly Quirke.

She added BMA is focused on finalising an agreement and will resume discussion with the unions.

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