FMG CFO Stephen Pearce and joint venture pipeline owners the Duet Group and TransAlta gathered in company of WA Premier Colin Barnett for the occasion.

With energy representing about 10% of the Solomon Hub's operating costs, Pearce said it was "terrific" that the operation was now 100% gas-powered.

"That's a really important achievement in terms of our ongoing environmental sustainability as an operation and as an organisation," he said at the event.

"It does take about 80 million litres of diesel fuel out of our system, which represents about 60,000 tonnes of carbon emissions."

The Fortescue River Gas Pipeline (FRGP) connects the Solomon Hub near Tom Price to the Dampier to Bunbury Natural Gas Pipeline.

It was built and is owned and operated by the Duet Group and TransAlta JV and received its first gas from TransAlta's 125 megawatt power station in September.

The pipeline has a 16-inch diameter and is expected to allow an initial gas consumption of up to 10 terajoules a day.

But Pearce said the pipeline had an ultimate capacity of 300 terajoules a day and was deliberately oversized to allow the future needs of FMG and potential third party users in the Pilbara.

"The logical extension is that we do move across to Chichester hub and then convert our power generation at Cloudbreak and Christmas Creek also to gas power generation," he said.

"But the biggest prize for us ultimately will be the conversion of the fleet to gas.

"And what a better place to do it that in the Pilbara in Western Australia where we have such abundant gas supply, right next door to the mining operations.

"There are lot of large energy users across the Pilbara and this is a terrific opportunity to create a large infrastructure footprint for the long-term benefit of the state."

WA Premier Colin Barnett congratulated all parties on building the pipeline, which is the longest to be built in the state in more than a decade.

Reflecting on the benefits it will have for the state, he said the pipeline represented the "obvious objective" of combining the strength of the WA's minerals and gas industries to achieve greater economic and sustainable benefits.

"It's an absolute win-win project," he said.

"It lowers the cost of energy for FMG's operations … uses natural gas domestically within WA, therefore replacing both the import and transport of diesel across our economy, and offers opportunities for expansion to other FMG operations and indeed to other parties operating in the Pilbara.

"So well done … on behalf of all those who have made it possible, building, owning and supplying the gas, and FMG who is the customer, I'm delighted to declare open the FRGP."