Australia's integrated energy company Origin Energy announced on Monday that China Petrochemical Corp (Sinopec) has agreed to increase a 10 percent equity interest in Origin and U.S. energy giant ConocoPhillips' joint venture Australia Pacific liquefied natural gas (LNG) project.

Sinopec has signed a agreement with Australia Pacific LNG that will see its stake in the project rise from 15 percent to 25 percent, Origin said.

"This agreement will reduce Origin energy's and Conocophillips' ownership interest to 37.5 percent respectively," Origin said in a statement on Monday.

Under the agreement, Sinopec will buy an additional 3.3 million tons a year of LNG through 2035 from the project.

That followed an earlier agreement for the sale of 4.3 million tons a year for 20 years from 2015.

The sale agreement would complete the marketing for the second train of the Australia Pacific LNG project, Origin said.

"The agreement with Sinopec represents another significant step for Australia Pacific LNG project and is testament to the strength of the project," Origin Chairman Kevin McCann said in the statement.

"To that end, we are well placed to make a final investment decision on the second train in early 2012."