Nearly 5000 websites have disappeared irretrievably after hackers broke into an Australian domain name registrar and web host.

Distribute.IT said that the attack had rendered four of its servers "unrecoverable", meaning that thousands of customers’ web content is lost forever.

"The overall magnitude of the tragedy and the loss of our information and yours is simply incalculable; and we are distressed by the actions of the parties responsible for this reprehensible act," Distribute.IT said in a statement.

The hosting company was attacked last Saturday, according to reports, but it is still unsure of the amount of data stolen and the method of the attacks.

Distribute.IT said that its data recovery team had been working around the clock, but that significant portions of data had been lost.

"We regret to inform [customers] that the data, sites and emails that were hosted on [servers] Drought, Hurricane, Blizzard and Cyclone can be considered by all the experts to be unrecoverable," it said. "Not only was the production data erased during the attack, but also key backups, snapshots and other information that would allow us to reconstruct these servers from the remaining data.

"Every effort will be made to continue to gain access to the lost information from those hosting servers, [but] it seems unlikely that any usable data will can be salvaged," the company said.