Unions fear thousands of job losses and pay cuts under a joint venture of Rio Tinto and BHP Billiton's Pilbara iron ore operations.

The Australian Manufacturing Workers Union said the joint use of rail lines and other mining services and infrastructure in the Pilbara could lead to a mass clear-out of train drivers, miners and other trades.

The mining giants employ about 20,000 staff and contractors across their Pilbara iron ore operations, which account for about half the world's seaborne iron ore trade.

AMWU secretary Steve McCartney said the workers would struggle to find new jobs after the retrenchment of at least 9000 other mining workers in WA since the downturn in September.

"The $12 billion in efficiency savings which they are talking about is just their code for job losses," Mr McCartney said.

Construction, Forestry, Mining and Energy Union secretary Gary Wood said there was no guarantee that workers who did keep their jobs would not have their pay cut.

He said the workers directly employed at both mining giants would have to break with their current industrial contracts and would be re-employed by a new entity, the joint venture, which was not legally required to maintain their wages.

But not all union leaders were concerned about trade and mining jobs. Australian Workers Union national secretary Paul Howes said retrenchments were more likely in senior management and the back office.

"I can't imagine large numbers of operational staff on the ground being terminated because they've still got to operate the same amount of facilities," he said. "The real cost savings here are in the paper economy rather than the real economy."

Rio Tinto spokesman Gervase Greene said it was too early to know what would happen with the number of jobs and wages.

BHP chief executive Marius Kloppers conceded there may be job losses, but said the number of positions would eventually grow.

The Chamber of Commerce and Industry expected that if the joint venture succeeded in unlocking more of the State's mineral wealth through the expansion of existing iron ore operations, or the development of new projects, then it would create more jobs in the long term.

Rio Tinto employee Dave Warrener said workers would be nervous about their jobs in the wake of the joint venture.

"In this type of climate with things changing like this it's going to have some result on people's work," he said after returning to Perth from Rio Tinto's Pannawonica site yesterday.

Mr Warrener said there was speculation that something was going to happen and surprise that the joint venture was announced so quickly.

KIM MACDONALD and ADRIAN WATSON