Woodside Petroleum insists its $4 billion gas and oil deal to drive future growth has come at a fair price despite weak oil prices.

The energy giant is buying a suite of oil and liquefied natural gas projects in Australia and Canada from US company Apache Corporation in one of its largest ever deals.

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Chief executive Peter Coleman said Woodside had timed the deal well, predicting a fall in operating costs after a 45 per cent slide in oil prices to current five-year lows of about $US61 a barrel.

However the price dive has prompted it to delay an investment decision on the proposed Browse floating LNG project off Western Australia’s northern coast.

Mr Coleman said the low prices meant it was a good time to be buying into the 49 per cent-built Wheatstone project, which is operated by global giant Chevron.

“We are now in a position to take advantage of challenging market conditions and use cash reserves and existing debt facilities to acquire very high quality assets,” he said on Tuesday.

“I don’t get too worried, I am really pleased not to be halfway through a project having committed at a $US100 oil to a cost structure and ending up with a $US70 world.

“It doesn’t matter whether it is $US100 or $US20 … what we need to do is develop a cost base that is commensurate with the price we expect to get to maintain the margin we need for the returns.”

Woodside, Australia’s largest pure play oil and gas producer, has long been criticised for its lack of a growth strategy to replace its existing North West Shelf and Pluto producing assets.

It will spend $US2.75 billion ($A2.98 billion) on the Apache assets, which will increase its oil and gas reserves by 20 per cent.

However Apache said it would net $US3.7 billion ($A4 billion) including reimbursement for capital expenditure from this year.