The NWSV argued that Synergy breached the terms of a long term take-or-pay contract signed in 2004 during a series of 24 deals with a third party, known as counterparty K, between 2006 and 2012.

Synergy had agreed to pay for a minimum quantity of gas each year, whether or not it needed it, with the quantity based on a minimal level of delivery plus 70% of the quantity by which the buyer's aggregate gas requirements (BAGR) for that year exceeds the minimum quantity.

The dispute between the parties concerns whether gas acquired by Synergy under the gas swap agreements should have been included in BAGR.

If it was, then Synergy will be obliged to pay a larger amount under the provisions of the agreement than it has paid to date.

Synergy argued that "purchased" meant the acquisition of title by payment of money or its equivalent on delivery, and that the gas swaps mean it did not actually acquire the gas it acquired under the gas swap agreements, except for swaps with immediate payment.

The NWSV argued acquisition of gas occurs when the gas is taken.

In a Supreme Court decision last week, Justice Rene Le Miere sided with big gas - Woodside, Shell, BHP Billiton, BP, Chevron and Japan Australian LNG - who own a sixth each of the NWSV.

Justice Le Miere said the utility had entered into the third party gas swaps and then failed to uphold the terms of the contract by taking for "valuable consideration" gas under the third party swap deals and not counting it as part of its overall gas requirements.

It means the utility, which posted a profit slump from $122.5 million to $57.1 million last year, may have to accelerate "make-up" payments

Justice Le Miere defined gas purchased as "gas acquired for valuable consideration", and so all 24 gas swaps needed to be included for the purpose of calculating Synergy's aggregate gas requirements.

He said the NWSV and Synergy should recalculate the gas swaps, and if they were not he would hear further submissions.