But the mineworkers' union has sounded a warning over the deal, saying expansion for expansion's sake had driven down prices in the past and would do so again if coal companies were not careful.

Two years of coal industry negotiations came to a head on Thursday when Newcastle Coal Infrastructure Group dropped its objections to a port expansion plan agreed by the State-owned Newcastle Port Corporation and Port Waratah Coal Services.

Coal services has Rio Tinto and Xstrata as major shareholders and runs the Kooragang and Carrington terminals.

The infrastructure group is led by BHP and is building the port's third coal-loader on Kooragang Island.

The coal plan gives the coal services the right to build another coal-loader, the port's fourth, on Kooragang Island, and it governs the access each "side" has to the other's loader.

"It's possible that coal exports from Newcastle could soon double from the current output of 91 million tonnes," coal services general manager Graham Davidson said yesterday.

The Construction, Forestry, Mining and Energy Union, which represents most of the Hunter's 10,000 mineworkers, backed the expansion deal but its national mining president Tony Maher sounded a note of caution this week when discussing the latest rush of foreign investors to the Hunter coal industry.

Mr Maher told mine delegates in Cessnock on Thursday that Chinese and Indian companies were investing in the NSW coal industry in the same way as Japan did in the 1960s and 1970s.

He said the union preferred Australian ownership but realised foreign ownership was inevitable.