Vast amounts of data stored by the Australian Taxation Office have been lost in the crash that brought down the agency's internal systems and customer service websites on Monday and Tuesday.

Large numbers of ATO officials were still sitting idle on Tuesday afternoon, unable to use their computers for a second day in the latest public service tech meltdown.

Fairfax has been told that it may be Wednesday before the Tax Office's systems are fully restored and that the ATO is trying to recover a petabyte – a million gigabytes – of data.

It is unclear what was recorded on the lost data but Tax Office management are adamant that no taxpayer information has been compromised.