The signing of the contract now brings the total program from 19 to 21 vessels.
The potential for the extension to the existing PPB contract was first announced last year, in Perth by Prime Minister Malcolm Turnbull and Timor-Leste Prime Minister Mari Alkatiri, and has now been formally signed with Austal.
The PPB program is designed to create inter-governmental co-operation to aid regional security in the South Pacific by helping to secure maritime borders in the region.
Austal CEO David Singleton said the program has so far met all milestones and the first PPB is scheduled for launch next month.
"The first PPB is on schedule for launch at the end of May with construction of the following two vessels already well underway with the fourth vessel commencing construction in June," Singleton said.
"Austal has a wealth of experience in shipbuilding having built over 300 vessels for 55 customers in 100 countries. We have utilised that experience and have met all major project milestones in the program on time."
Austal is hopeful that the program, which is supporting 200 jobs at Austal and an estimated 200 jobs in the industry, is the starting point for exporting the Guardian-class vessel.
"This program will support 200 direct jobs at Austal and a further 200 indirect jobs in the broader industry. This is in addition to several hundred jobs supported at Austal by a number of large commercial export contracts currently under construction and planned for construction at our Henderson shipyard," Singleton explained.
"We are working on other export opportunities for the Guardian-class patrol boat thereby helping Australia sustain its sovereign naval shipbuilding industry which has already delivered over 40 patrol boats to international customers."
The Pacific Patrol Boat contract was awarded to Austal in May, 2016 and is worth $305 million for the original 19 vessels and associated in-service support.