Melbourne's water wholesaler collected an extra $458 million in revenue from "increased water, sewerage and drainage prices" last financial year.

Revenue surged from $1.258 billion 2012-13 to $1.716 billion in 2013-14.

Much of Melbourne Water's extra revenue was attributed to flat revenue collection in 2012-13 because of a price cap linked to previous overcharging of customers for its desalination plant.

The wholesaler reported a $90 million profit last financial year and sent a $188.3 million dividend to the Napthine government in the state election year.

The 2013-14 Melbourne Water annual report showed the first full-year operation of the Wonthaggi desalination plant cost $650 million, or about $1.8 million a day.