"Another boat arrived to pull him out of the water and then it was noticed that the lady had suffered some severe and fatal injuries."
The other boat got between the uninjured diver and the shark, and he was able to get back into his boat and pull the woman from the water.
Insp Mulligan said the other boat was 5.3m long and the three fishermen onboard believed the shark was longer than their vessel.
The Department of Fisheries believes the shark is more than 3m long and officers have set gear to catch and kill the animal.
Garmah Jones has a boating business in the area and told AAP he wanted to see laws changed to allow fishermen to catch sharks.
"People are losing their lives. They need to do something," he said.
"It's unusual for sharks to be here, but they have been coming."
The City of Wanneroo has closed beaches either side of Mindarie boat ramp and beaches will remain closed through to Monday.
Surf Life Saving WA had tweeted a public report of a shark sighting at 11.30am, 1km offshore from Mindarie Beach.
Two hours later, they said a 4.5m great white shark was spotted 300m offshore from Mindarie marina.
It is the second shark attack fatality in Western Australia in less than a week, although the locations are more than 100km apart.
Ben Gerring, 29, died in hospital on Friday night after being mauled by a shark at Gearies break in Falcon, south of Perth, on Tuesday, leaving behind his pregnant fiancee Jasmine Boyer.
A large shark was caught and killed on Wednesday in baited drum lines close to the attack site.
A GoFundMe page was set up to help Mr Gerring in his recovery but people have continued to donate money since his death, raising more than $43,400.
Erika Magness posted: "Ben you were the kindest soul and you will never be forgotten. You have left such a beautiful gift - your's and Jasmine's baby."
Mr Gerring's father, Shane, described those who tried to save his son as "angels of the ocean" and told The Sunday Times newspaper he opposed a shark cull.
- AAP
