More than 800 ducks were shot and their carcasses left to rot in the water, the worst bird massacre gaming authorities had seen in decades.

There were 104 endangered freckled ducks - one of many rare species - killed that morning near Boort in the state's north-west. Swans, whistling kites, and other species that were obviously not ducks, were also targets. Twelve months later - and as a fresh duck season opened this weekend - not one person has been charged.

The government agrees we may never know who was involved, or what efforts were made to bring them to justice.

A full report into the so-called Box Flat massacre is believed to name key suspects and witnesses, along with details of the department's nine-month investigation.

But despite repeated requests, the government is refusing to release the documents, insisting it contains private material and could compromise the process.

Agriculture Minister Peter Walsh said that last year's illegal shooting was unacceptable, and that in the lead-up to this season, Game Victoria had been working with hunters ''to reinforce that hunting is a privilege, not a right''.

But he added: ''It is not appropriate for Game Victoria to release sensitive personal information relating to this ongoing investigation.'' But others are not convinced. The Coalition Against Duck Shooting's spokesman, Laurie Levy, said locking away the results of the investigation was not in the public's interest, but it was in the department's interest.

Last year's massacre was not the first, and even before the event a hunter, acting as an informant, told a Department of Sustainability and Environment official that Box Flat should be watched on the duck season opening weekend, a warning Game Victoria ignored.

The refusal to release the full report has also sparked renewed claims of a ''cover-up'' by those who believe it was never in the government's interest to pursue the matter, particularly given that game hunting - which generates $100 million annually and includes more than 25,000 licensed duck hunters in Victoria - is such big business.

The duck hunting season continues until June 9, with a limit of 10 ducks a bag.