Last week's announcement of a $238 million profit did not take into account the impact of the transfer of the debt-laden Tamar Valley power station and write-down of dam infrastructure on the bottom-line.

The state-owned business's annual report, tabled in Parliament yesterday, reveals the acquisition of the gas-fired power plant resulted in a $335 million impairment.

Hydro was forced to take the unprofitable asset off Aurora's hands as part of the state government's energy market reforms.

The value of hydro generation assets was also revised down by $118 million in 2013, reflecting plans to scrap the carbon tax.

The state Opposition accused the government of misleading the public about the state of the business.

"This is a $330 million government stuff-up laid bare," Liberal energy spokesman Matthew Groom said. "What makes it worse, is that last week the government tried to pull the wool over the eyes of the Tasmanian people to cover up this monumental government stuff-up."

Hydro has not run the Tamar Valley power station for more than three months.

However, Energy Minister Bryan Green said it was an important asset.

"Already, since taking ownership of the station, Hydro Tasmania has minimised its generating costs by not running the station through a period of higher than average winter rainfall," Mr Green said.

"The station remains on standby and will resume operation whenever needed."

The government is reviewing the situation and may reallocate some of the debt associated with the power station to other state-owned businesses.

Hydro chairman David Crean said the impairment did not affect its operating cash position or its returns to government.

More than $263 million is expected to be pumped into the state government's coffers thanks to Hydro's performance.