Mr Tatlow says he initially thought he saw an overturned hull. "A minute or two later it squirted water and I realised it was a whale," he said.
"I know enough about whales, it was a right whale and I got the binoculars on it and shortly after I saw a reddening in the water around it and then a baby whale appeared. This right whale had given birth."
Mr Tatlow says three members of another family also saw the birth.
A southern right whale was born off Taroona Beach last month, the first reported birth since commercial whaling ended in the mid 1800s.