"I was actually filming it as it was swimming," Ms Radley said to The West Australian. "It was feeding then swam quickly to the shore and beached itself."
When Ms Radley shouted for help and no one stepped up, she herself rushed towards the water and grabbed the black marlin. Soon, two other bystanders got up to help her.
Together, the three managed to guide the big fish back into the sea.
Footage of the rescue was captured and shared on Facebook by the blog, 1 Journey 1 Life 2 People.
You can watch it below:
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