Garrett gave approval to Australia's largest liquefied natural gas project under the Environment Protection and Biodiversity Conservation Act.
The Western Australian government gave environmental approval to the project on Barrow Island earlier this month.
"I am satisfied that, with the conditions placed on this project by the WA government and the strengthened conditions I have imposed, the expansion can proceed without unacceptable impacts on the matters covered by commonwealth jurisdiction in relation to this project, namely listed threatened and migratory species," Garrett said.
"I have given this proposal the same high level of scrutiny that I apply to all of my decision-making responsibilities, as I am legally required to under national environment law.
"While the economic benefits of this project clearly cannot be overlooked, and legally must be a factor in my consideration, my key focus has been whether or not this expansion could adequately mitigate and manage any additional potential impacts on the protected species, such as the flatback turtle and the terrestrial fauna."
Garrett added that he had also agreed to vary the conditions of the original 2007 approval to ensure the strict protection requirements to the expansion proposal would apply to the entire operation.
"Additionally, to increase transparency, state and federal requirements will be incorporated within single management plans.
"This will give the public a clearer view of how the company plans to manage or avoid potential environmental impacts.
"I have also made it a requirement for Chevron to make publicly available its performance reports, which will outline how the project is complying with the conditions of approval."
Chevron must now prepare a range of plans relating to the protection, management and monitoring of nationally protected terrestrial fauna for approval.
The company must also meet a number of requirements to deal with any potential impact on the flatback turtle.
These include setting up a monitoring program to measure and detect changes to the turtle population, and to outline the measures and controls in place to manage and prevent any impact on the turtles, particularly the reduction of light and noise emissions.
"In setting these conditions, I took account that Chevron is required to contribute $62.5 million to the northwest shelf flatback turtle conservation program to increase protection of the turtle population over the life of the proposal," Garrett said.
"I have also imposed a condition which will allow me to direct Chevron to take action if I believe the project is adversely impacting on the flatback turtle."
Chevron's Gorgon project is estimated to generate billions of dollars in royalties and about 6000 jobs, with more than 3500 direct and indirect jobs sustained throughout its life.
It will be capable of producing 15 million tonnes per annum of LNG through the development of the Gorgon and Io/Jansz fields that will be linked by subsea pipelines to Barrow Island where the three-train LNG plant will be located.
It will include a 300-terajoule per day domestic gas plant and pipeline to the mainland, as well as a carbon dioxide sequestration project aimed at storing the high CO2 levels present in Gorgon gas in deep formations beneath Barrow Island.
Chevron expects to make a final investment decision on the project in the coming months.
The Gorgon partners are Chevron Australia (operator and 50%), Shell (25%) and ExxonMobil (25%).
