The Opposition is seeking answers from Hydro Tasmania as to why a massive loss was not disclosed as part of the government business's financial results announcement.

Last week, Hydro's bosses called a media conference to announce a record pre-tax profit of $238 million.

In its annual report tabled yesterday, the state-owned company reveals its final result is a pre-tax loss of almost $250 million.

Liberal energy spokesman Matthew Groom says he will write to chairman David Crean seeking an explanation.

"There is no question that there was a material omission in the results announcement by Hydro Tasmania and the Government last week," he said.

"It raises a very serious question in my mind and it's a matter of deep concern to me."