A huge new oil deposit off Western Australia is being heralded as the largest discovery in over 30 years.

Analysts say there are about 300 million barrels of oil in the field.

US firm Apache was drilling for gas, not oil, when it stumbled upon the field in the Canning Basin off the north-west coast of WA.

Four companies have a stake in the find – Apache (40%), Carnarvon Petroleum (20%), Finder Exploration (20%) and JX Nippon (20%).

But it is still early days, and the company may only be able to recover a portion of the total amount.

Analysts say they have not worked out how the easily the oil would flow from wells, and how to actually develop the remote offshore field.

Based on data from the one well dug so far – it appears at least 60 million barrels of recoverable oil are on offer, though new wells will be sunk as soon as possible.

All of these issues will be weighed against the cost of developing and moving the oil, along with ongoing studies of how much is actually there.

But despite the uncertainty, the find has already sent stocks in Apache’s drilling partner Carnarvon through the roof, bringing in an extra $157 million in market value.