An incredible shark attack on a marlin has been filmed by one of Australasia's best-known fishing writers and photographers - and he knows he could have been the victim.

Al McGlashan was in the water off Port Stephens, 200km north of Sydney in New South Wales, to film the tag and release of the marlin when the mako shark swooped, tearing at the body of the hooked gamefish metres away from him. He is convinced the shark moved in only after McGlashan, who had been taking close-ups of the action, moved back to take panoramic pictures of the marlin's release.

"That shark was there watching me the whole five minutes I was in the water, and what is even more impressive is that it waited till I moved out of the way before attacking the marlin," McGlashan told the Herald on Sunday.

Before jumping into the water to film the tag and release, when the marlin had been hauled in close to the catch boat, McGlashan and his crew had marked a second fish on the fish finder, which they thought to be another marlin.

However, he now believes it was the shark stalking the marlin. "In all my life I have seen some amazing things - including a few shark attacks - but never have I been witness to such a spectacle from outside the safety of the boat.

"As sad as it was to see the marlin eaten, seeing nature in its most raw form and capturing it all on film, both stills and HD footage, was a once-in-a-lifetime experience."

He estimated the shark would have weighed about 270kg. At first the crew on board had no idea what was going on and couldn't work out what all the commotion was about.

With no chance to get to the boat, McGlashan figured he should just shoot the whole thing. All he could remember thinking during the attack was, "Man, that is one massive shark."

McGlashan is one of Australasia's most respected fishing writers and photographers. He is a regular visitor here and is a contributor for New Zealand Fishing World magazine.

He has written a report about this attack, which includes a series of images, for the latest issue of the magazine, which hit the shelves this weekend.