AUSTRALIAN scientists are baffled as to why enormous Queensland groper fish are washing up dead on North Queensland beaches without a scratch on them.

The Townsville Bulletin reported today researchers have determined the Queensland gropers - the largest reef-dwelling fish in the world - were dying from septicemia caused by bacterial infections but are at a loss to explain why, a Fisheries Queensland spokeswoman said.

Mystery surrounding the deaths continue after a 1.7m groper washed up on Saturday morning at Rowes Bay, a Townsville coastal suburb 1363km north of Brisbane, Queensland.

Field Officer Jeremy Thompson said he and officer Vaughn Heath were alerted to the gargantuan fish on the beach by residents from a nearby caravan park.

Mr Thompson said the public then helped the officers get the fish on to the back of their truck and they brought it back to their office to freeze it for Biosecurity Queensland to collect and research.

"The members of the public who saw it seemed a bit excited about the size of it," he said.

"It would have to be about 70kg."

A fisheries spokeswoman said although it was rare for the monster fish to wash up in Townsville, it was common in the far north.

The problem became so bad between Cairns and the Daintree, the Department of Primary Industries and Fisheries began researching why the problem occurred.

In the past two years, more than 50 gropers have washed up dead on North Queensland beaches, mainly north of Cardwell, all infected with the bacteria Streptococcus agalactiae.

Read more at the Townsville Bulletin