The sales and purchase agreement follows a heads of agreement signed between the two parties on December 21, 2006.
The deal lifts the Woodside-led JV's LNG sales to about 1.5MMtpa after 2009.
Osaka Gas has been a major customer of the NWSV since 1989.
The six equal partners in the NWSV are Woodside Petroleum, BHP Billiton, BP, Chevron, Japan Australia LNG and Royal Dutch Shell.