The NSW government has raked in more than $9 billion from the sale of government land, including schools, while the state's schools are grappling with an overcrowding crisis, Labor says.

Nearly 400 schools have been sold by the government since it came to office in 2011, a budget estimates hearing revealed on Monday, despite a $775 million school maintenance backlog and widespread overcrowding.

“It is completely embarrassing that the greedy cash grab of this government has been so short-sighted as to ignore the need for schools for our growing population,” shadow education spokesman Jihad Dib said in a statement on Tuesday.

The Department of Education might need to buy back property it had sold to make up the shortfall of schools across the state, Property NSW chief executive Brett Newman told the hearing on Monday.

“I cannot sit here and give you a guarantee that the Department of Education may not need to buy back any asset the government has previously sold in the past,” Mr Newman said.

He confirmed the government had sold 384 Department of Education properties, with Labor saying 162 of those were in the past year.

There was another $1 billion of property sales “in the pipeline”, the hearing was told.

In 2014 the government made the controversial decision to sell almost 300 state-owned properties in Sydney’s prime harbourside real estate locations near The Rocks to raise up to $500 million.

Another 140 properties are yet to be sold at Millers Point, the hearing heard.

The sell-offs leave an “intergenerational burden for the future”, Labor’s finance spokesman Clayton Barr said.

“The mass sell-off of publicly-owned properties is intergenerational theft from our past, and an intergenerational burden for our future. This government can’t even see 2019, never mind be able to see past it,” Mr Barr said.