The annual phenomenon is taking place in the waters off Magnetic Island, Queensland, and will last just a few days.
This year's event is the 45th anniversary of the first recorded mass synchronised coral spawning in 1981.
In doing so, Professor Peter Harrison and other researchers used their observations to transform scientific understanding of coral reproduction.
This year also marks 45 years since the Great Barrier Reef was added to the UNESCO World Heritage List.
During the event, corals simultaneously release billions of egg and sperm bundles into the ocean, creating the next generation of corals and sustaining reef ecosystems.
Witnessing the phenomenon was a career-defining moment for Prof Harrison, who has since dedicated his career to improving understanding of coral reproduction.
"I vividly remember the first time I saw mass coral spawning at Magnetic Island," he said on Saturday.
"I was overwhelmed with the scale of the event.
"So many different types of corals were releasing eggs and sperm together and the eggs sperm bundles filled the area and floated to the surface creating one of nature's greatest underwater spectacles."
Inspired, Prof Harrison sought to apply his newfound understanding to regenerating corals at scale on damaged reefs.
This led to the development of coral IVF, which uses the natural reproductive process of corals to generate larvae in floating pools for relocation to damaged sections of the reef in order to stimulate recovery.
His technique has been used to renew the Great Barrier Reef and others around the world.
The annual spawning is an important reminder of the reef's ability to continuously create life and of its significance as the world's largest reef system.
"The Great Barrier Reef is a world heritage icon and it's Australia's responsibility to manage it effectively," Prof Harrison said.
"I hope people recognise when they see the Great Barrier Reef just how important it is nationally and internationally because it is the largest coral reef system on our planet."
The event comes amid concerns from locals that the Port of Townsville's plan to dump silt and sediment at sea would negatively impact reef reproduction.
The Port of Townsville says it will not dredge until November 4.
At this point, it will dump sediment build-up 6km from Magnetic Island's shores.
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