Sharks, coral and other marine life have been found entangled or starved to death in a 5km-long, 14-tonne "ghost net" believed discarded from an illegal fishing vessel at the Rowley Shoals.
Mermaid Reef, where the net was found, has been under protection by the Commonwealth since 1991 and is part of marine reserves announced this month by Federal Environment Minister Tony Burke.
A customs vessel spotted the net at the reef several weeks ago.
Broome brothers Chaz and Chris Kennedy, who operate the Karma IV fishing charter vessel and have a contract with the Australian Fisheries Management Authority, acted after getting a call to assist.
After a 15-hour overnight trip from Broome, the professional divers arrived to find the net drifting.
It was bundled but sections had broken off and spread, strangling corals over about 1000sqkm.
Chaz Kennedy said had the net fully unfurled, the effect on the environment would have been catastrophic. Luckily, it appeared to have been in the water only for a few weeks, because of the lack of marine growth on it. He said the design showed it came from Asia.
"It had been estimated at two tonnes but it would have to have been pushing 15 tonnes," Mr Kennedy said. "When you approached it diving, you could see the lead line and the net all tangled among the coral rises."
As the divers worked to untangle the net over eight hours, they found a reef shark had become trapped after eating something stuck in the net. "We had to pick the net out of its teeth underwater," Mr Kennedy said.
It took 38 hours to tow the stinking net back to Broome.
Department of Fisheries marine officers inspected the net to make sure it held no introduced pests, then it was destroyed.
Mr Kennedy said ghost net sightings had increased as governments "turned a blind eye" to illegal fishermen to focus on boat people. <div class="endnote">