Dugongs and snubfin dolphins have died after being caught in commercial fishing nets in northern Queensland waters, authorities have confirmed.

Key points: At least two snubfin dolphins and four dugongs killed since September

At least two snubfin dolphins and four dugongs killed since September GBRMPA only released information after media inquiries

GBRMPA only released information after media inquiries WWF Australia calling for more net-free zones

The Great Barrier Reef Marine Park Authority (GBRMPA) said two snubfin dolphins drowned after being caught in a commercial fishing net operation in October.

"The fisher who caught the dolphins followed all required fishing rules and protocols, including attendance of the net and reporting of the incident," a GBRMPA spokesperson said.

In September and October, four dugongs were found dead in Bowling Green Bay near Townsville, with at least one killed by a commercial fishing net.

GBRMPA said a second dead dugong was found floating in close proximity to a commercial netting operation, while the other two carcasses in the area were too decomposed to determine a cause of death.

The authority only released the information in response to a media inquiry and has refused to reveal where the snubfin dolphins died, due to privacy concerns.

It also said it could not release photographs of the dead dolphins for privacy reasons.

The snubfin dolphins were recovered by marine park officers and taken to a university for research and examination.

"Some of these unfortunate marine mammal deaths relating to net fishing reinforce the importance of mitigating risks and ensuring ecologically sustainable fishery management arrangements are in place adjacent to and throughout the Great Barrier Reef Marine Park," the spokesperson said.

'Barrier Reef needs more net-free zones'

The gillnet deaths have outraged conservationists.

Gillnets are long rectangular nets which are set horizontally along the ocean floor.

WWF Australia head of oceans Richard Leck said more had to be done to prevent such tragic outcomes.

"This is a really tragic incident to hear about. These snubfin dolphins are a rare Australian species they are only found in Australian waters and they're a vulnerable species," he said.

Four dugongs have been found dead near Townsville since September. ( stringer, file photo: Reuters )

Mr Leck said many gillnet deaths were not reported.

"We think it's actually the tip of the iceberg."

He said WWF Australia was calling for the establishment of an 85,000-square-kilometre net-free zone in north Queensland.

"We're calling for the major parties to commit to more net-free zones in the Great Barrier Reef Marine Park, starting with a large net free zone in the north of Queensland."

Mr Leck also encouraged GBRMPA to be more transparent and to keep people updated on when marine mammal deaths occurred.

"We need this information to be in the public realm to inform people what's happening and to get those best solutions to protect these very vulnerable and incredibly charismatic species into the future."

GBRMPA said there were no further investigations into the deaths as fishers followed all reporting protocols.

It also said the deaths were reported with the Department of Environment and Heritage Protection and made public in annual reports.