Perthnow says that maybe he was kitesurfing.

Kite surfing at 5 am?

According to Perth Now the man was wearing a wetsuit, has been identified and the circumstances are not suspicious. It is unclear whether he was kiting or surfing - apparently emergency services at the scene speculated he may have suffered a heart attack or hit his head on the reef. A "witness" believed the man had been kitesurfing:
www.perthnow.com.au/news/wa/mans-body-washed-ashore-at-city-beach-ng-32f8b910040ad8ea51a52f77ba4d6b84

WA Today indicated a kite was found nearby:
www.watoday.com.au/wa-news/passerby-discovers-body-at-city-beach-20160627-gpsjo2.html

I wonder whether he still had his harness on or whether he ditched it ...

gday wishy
i'm friends with someone who went to the funeral last week. he was a young man originally from down south ways. not much word on any of the details, he was fairly active in surf/distance swimming to my friends knowledge, and that is about all that is actually available. no definitive answer on whether it is kite related either, but sounds unlikely or really unusual if so. he wasn't known by that group of friends as being a kiter - but i didn't know him myself.
probably best to wrap up speculation, as coroner's inquest is at least a year away from having findings apparently.
pretty sad/introspective time for the friends and family, as with all unexpected passings i guess... RIP