The blowout of an oil well in the Great Australian Bight could leak for more than 100 days and foul beaches along Australia's coastline from Albany in WA to Sydney's beaches and beyond, a leaked document shows.

The draft oil pollution emergency plan for Norwegian oil giant Equinor's planned Stromlo-1 exploration drilling in the Bight, was obtained by Greenpeace. The proposed drilling is in waters about 370 km from the nearest coastline at water depths of 2239 metres.

Australia has had offshore oil drilling woes in the past, such as PTTEP's West Atlas oil rig blaze off the Kimberley coast in 2009.

Modelling for the "loss of well control" assumed average daily release of 6739 cubic metres of oil, creating a slick of "potentially high concentrations of surface oil" near Port Lincoln and Kangaroo Island in South Australia, Port Fairy in Victoria and King Island in Bass Strait.

So-called weathered oil - hardened tar balls of more than 10 grams per square metre after part of the fuel evaporates - would eventually reach as far up the NSW coast as Port Macquarie north of Sydney.