VICTORIANS will pay an average $570 million a year for the next three decades to have the southern hemisphere's largest desalination plant - even if no water is needed.

Detail buried in a barrage of reports tabled in Parliament on Wednesday contained highly sensitive figures that the Brumby government has fought to keep secret: the real cost of the massive Wonthaggi plant, one of the largest public-private partnerships undertaken worldwide.

The Department of Sustainability and Environment report shows that in cash or nominal terms taxpayers will pay at least $15.8 billion to the Aquasure Consortium to operate and maintain the plant for 28 years after it begins operation at the end of 2011.

According to the government's calculations, the so-called water security payment is equivalent in today's dollars to $4.6 billion. In all, the government says the plant will cost $5.7 billion in today's money for the construction and operation of the site.

Revelation of the payments comes at a difficult time for the government in a surprisingly wet election year that has allowed it to ease water restrictions but raised tricky questions about the cost of major water projects including desalination and the contentious north-south pipeline.