The troubled ATO website has crashed again. Picture: News Limited

THE Australian Taxation Office has been forced to apologise again for its troubled online service, with the website down today following four days of maintenance.

The ATO issued the apology through its Twitter account this morning, saying it was “investigating current issue with our website as a priority”.

The website went back online about 10.30am after being offline for about two hours.

People who tried to access the ATO online this morning were met with a basic message saying there was an “unknown error”.

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The ATO later updated that error message saying the website is “currently not available”.

The ATO announced last week that the website would be unavailable over the four days of Easter break in order to carry out system maintenance.

The ATO website suffered a massive outage in December last year and again in February. After the December system failure, the ATO was scrambling to recover the equivalent of 20 million four-drawer filing cabinets of data.

The system maintenance over the weekend was planned to give the ATO technical staff time to upgrade the system from the problematic storage area network, provided by Hewlett Packard Enterprises, that caused the recent failures.

An ATO spokesman said the wesbite was back to “full functionality” after a “short disruption”.

“The issue was not related to the IT systems work carried out over the Easter long weekend to install our new storage area network which was successfully completed,” the spokeman said.