BHP has awarded a major steelwork contract for its new South Flank mine in the central Pilbara to a Chinese company, drawing the ire of the Australian Manufacturing Workers' Union.

Up to 20,000 tonnes of manufacturing and fabrication work for the new $US3.6 billion ($A5.0 billion) mine has been awarded to Tianjin BOMESC Offshore Engineering Company, based in northeast China, a move the union says is depriving WA workers of jobs.

“Instead of supporting WA industry, apprentices and jobs – BHP has sent opportunities offshore,” AMWU WA State Secretary Steve McCartney said in a statement on Tuesday.

“This company profits off our finite one-off resource and without upstream manufacturing we will get nothing back.”

But the mining giant says the decision to send the work offshore was “necessary to ensure the project is delivered on time”.

“All Western Australian companies were invited to provide an expression of interest for all assembly and fabrication packages,” a BHP spokeswoman said.

“BHP expects approximately 75 per cent of total value of all steel works associated with the South Flank project will be undertaken in WA by local steel manufacturers and workers,” the spokeswoman said in a statement.

The construction of the South Flank mine was announced by BHP in June and will replace production from its 80-million-tonnes per year Yandi mine in the state, which is reaching the end of its economic life.

Production is expected to start in 2021 and run for more than 25 years.

By Benita Kolovos