The Wheatstone project, being constructed at Ashburton North about 12 km west of Onslow, comprises two LNG trains with a combined capacity of 8.9 million tonnes per annum and a domestic gas plant.
Under the terms of the agreement, the Wheatstone will supply one million metric tonnes of LNG per year to Chubu for up to 20 years.
The partners of Wheatstone project include Chevron, Apache Energy, Kufpec, Shell and Kyushu Electric.
Earlier, Wheatstone partners had announced long-term agreements to sell LNG to Tokyo Electric Power and Kyushu Electric Power.
Apache Australian is a subsidiary of oil and gas exploration and production firm Apache Corporation.