Woodside Petroleum has surprised the market with a super-sized $2.5 billion equity raising as it asks investors to back a revamped LNG expansion initiative in Western Australia, despite worries about costs and a top-heavy weighting in the portfolio toward WA gas.

The share sale is more than double the size needed for the acquisition of ExxonMobil's half-share in the undeveloped Scarborough field off the WA coast, a deal that will give Woodside majority control with a 75 per cent stake.