Ive seen a few beasties out there,
The latest and largest was a ray. definately an 'offshore' species that had swam in for a sunbake between fisho and possos in torquay.
the wing span had to be over 3 metres, in 4-5 ft of water (board had a 14inch fin on!)
I was praying there would be enough clearance and it wouldnt go me as I wasnt quite on the plane at the time.
Another beastie was the barwon river dolphin archie 2 years ago! He surfed off my bow wave for 30 seconds or so - that was awesome

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