Mark Sullivan.

Mark Sullivan. Photo: Lyn Mills

Canberra's water utility has admitted under-reporting the wages of its chief executive by nearly a quarter of a million dollars.

ACTEW Corporation reported the annual pay package of chief executive Mark Sullivan at $621,000 in its 2010-11 annual report when the actual remuneration package was $855,000, including superannuation.

But the taxpayer-owned water company says the ''stuff-up'' was simply a reporting error and had no effect on its bottom line.

ACTEW agreed in 2010 to publish the salary of its chief executive after suppressing details of its executives' pay for five years.

Treasurer Andrew Barr told the Legislative Assembly on Tuesday that he and the Chief Minister, who are both the official shareholders in the company, were looking for answers.

''I've recently been notified by the chairman of the ACTEW Corporation that an error has been discovered in its 2010-11 annual report,'' the Mr Barr told the chamber.

''I'm advised that the corporation discovered that they had understated the executive remuneration of the managing director in the order of $234,000.

''The correct salary should have read $855,588. This correct salary is consistent with the figure reported and tabled in the 2011-12.''

Mr Barr said he was still seeking details of how the mistake was allowed to occur, saying he was ''surprised and disappointed'' by the discrepancy.

ACTEW chairman John Mackay said the mistake had been discovered as the company prepared to appear before the Assembly's estimates committee.

''We had people preparing for estimates committee which is coming up in some months time,'' Mr Mackay said.

''One of our people was preparing and they looked at this and said this has got to be wrong. And we went back and checked, and it was wrong.

''We went back to seek legal and accounting advice and the advice was that in a technical accounting sense the mistake was not material.

''We found the error - the error was a straight stuff-up.''