Brian Guest, 51, was diving for crabs with his son at their local beach near Rockingham, 30km south of Perth, when he disappeared about 7am. His son, Daniel, 24, swam ashore and raised the alarm.

Witnesses saw fins and blood in the water.

''Something very traumatic and pretty violent has happened there and we are treating it as a probable shark attack ,'' acting police inspector Mark Valentine said.

The three emergency service aircraft, 10 boats and divers conducted a grid search but by late yesterday had found nothing. The search will continue today.

Family spokesman and police officer Steve Kent said Mr Guest was a devoted husband and father who knew the dangers of swimming in the ocean.

''[He was] an avid fisherman, an avid diver and a loving husband and father, he said. ''His son and he were out in the water, which he had done quite regularly. Unfortunately, this morning there was a shark in the water and it's taken the father.

''At the time his son didn't actually sight what had happened but there were some people on the beach that saw and heard some things in the water.''

About 4000km away in Sydney, kayakers Justin Stanger and Steve Kulscar were lucky to survive, with one of them falling from his craft as a 5m white pointer circled them about 1km off Long Reef.

Fisherman Glenn Morgan was in a runabout nearby when he saw a large fin break the water near the kayakers, about 11am. ''All of a sudden I saw one bloke topple off his kayak, so we cut our anchor rope and went straight over to him and got him in the boat,'' MrMorgan said last night.

The shark came perilously close as the two kayakers fastened their craft to the tinnie. It circled for at least another 10 minutes before leaving.

''The adrenalin was just pumping,'' Mr Morgan said.

''I reckon they would have died if we weren't out there ... the shark was pretty hungry.''