Alright so check it out... I'm no marine biologist but take a look at the attached image which demonstrates Perth's "Shark Attack Zone" to figure out where in Perth a shark attack may be more likely to occur. Every major attack in Perth has occurred in this Shark Attack Zone with the exception of a couple of bull shark attacks in the Swan River back in the day.
In the attached Google Earth image, the white line marks the Shark Attack Zone and the yellow thumb tacks show the locations of the previous attacks at Cottesloe, Rockingham and Garden Island. The Shark Attack Zone forms part of a reef habitat that stretches from Rockingham to Garden Island to Rottnest Island to Cottesloe. Given the high frequency of attacks in this area it is at least arguable that is less safe to enter the water in or near the Shark Attack Zone.
Also the fact that the mouth of the Swan River is only a stones throw from Cottesloe may also increase the likelihood of attacks as well as the fact that it is currently the migratory period for whales during the months of September to December. The Estuary in Mandurah being in relatively close proximity to the Shark Attack Zone may also indicate a higher likelihood of attack in Mandurah.
Regardless of all this theorising... the chances of being attacked in the Shark Attack Zone are still extremely remote but anything is possible. In reality you can be attacked in WA waters anywhere and at anytime. You either accept the risk or don't surf just like you accept the risks to drive on the roads.
I think I'll avoid Cott and Mandurah this Weekend! Peace out!