The miner said it's the next major step in the development of the new mine site, which is set to create more than 3,000 jobs.
BHP's board approved US$2.9 billion in capital expenditure for the project in June of this year.
Back then, BHP president operations, Minerals Australia, Mike Henry, said: "South Flank is a capital efficient project, which offers attractive returns, and which was approved following a thorough evaluation under BHP's Capital Allocation Framework.
"It will enhance the average quality of BHP's Western Australia Iron Ore (WAIO) production and will allow us to benefit from price premiums for higher-quality lump and fines products."
The South Flank project expands the existing infrastructure at Mining Area C, and involves construction of an 80Mtpa crushing and screening plant, an overland conveyor system, stockyard and train loading facilities, procurement of new mining fleet and substantial mine development and pre-strip work.
First ore from South Flank is targeted in the 2021 calendar year, with the project expected to produce ore for more than 25 years.