The extension is for 10 months and is worth about $53 million.
Australia's F/A-18 Hornets have received deep maintenance services from BAE Systems since 2003. The company worked with L-3 MAS Canada is 2009 for the Hornet's structural refurbishment program.
Under the contract extension, 14 more F/A-18s will be modernized.
"This contract change is the culmination of many months of hard work to deliver an optimized Classic Hornet Deeper Maintenance and Modification Support program," said BAE Systems Director, Aerospace John Monaghan.
"Most importantly, the contract incorporates additional work while delivering program efficiencies and real savings aligned with (the Department of) Defense's Strategic Reform Program. The additional work also secures jobs for our technicians and support staff at our Williamtown facility until October 2013."
L-3 MAS President Sylvain Bedard said: "We see ourselves providing the heavy lifting on the (Australian air force's) Classic F/A-18 Hornet program. This contract extension demonstrates the commonwealth's confidence in our team to deliver Hornet support into the foreseeable future."