"A-HA, so this is where I am" - those could be the thoughts of this humpback as it speared out of the water, perilously close to a sailing boat, in Sydney Harbour.

Daily Telegraph fishing columnist Al McGlashan was returning from a trip on Tuesday afternoon when he noticed some "rather large splashes" near pleasure boats just inside South Head.

"I picked up the camera just in case and right away the whale came out of the water again," McGlashan said.

"It wasn't one of the big sideways breaches you see a lot, but more just coming straight up and up and up, out of the water.

"It hovered there for a moment ... like the whale was trying to get its bearings. Then it was as if it figured out where it was and swam out through the Heads."

McGlashan said very few whales had been seen off Sydney in the past week, after hundreds had passed the city on their southern migration in late winter and early spring.

National Parks and Wildlife has issued a plea for boaters to be careful of the large numbers of baby humpbacks and their mothers swimming unusually close to shore during this year's southern migration.

Originally published as Moby Dick moment on the Harbour