The half taxpayer-owned company says the salary package of chief executive Michael Costello, believed to be more than $1 million a year, will remain under wraps despite the recent row over executive pay.

Not even Chief Minister Katy Gallagher or Treasurer Andrew Barr, the two shareholders in ActewAGL's 50 per cent owner ACTEW, have been told of Mr Costello's remuneration.

The main trade union representing the ActewAGL workforce has described the chief executive's pay deal as ''the monster in the cupboard''.

Controversy over executive pay in Canberra's utilities sector has been raging since March 20, when the government announced that the $855,000 pay packet of ACTEW managing director Mark Sullivan had been under-reported by $234,000.

The political storm culminated on Monday with the resignation of ACTEW's veteran chairman, John Mackay, and Mr Sullivan agreeing to a new contract worth $140,000 less and the removal of his bonus arrangements.

ActewAGL is a joint venture between ACTEW, a taxpayer-owned water and sewerage corporation, and two private companies, Australia's AGL and Singapore-owned Jemena, which controls nearly all of the capital's electricity and gas distribution.

The joint venture has more than $700 million a year in sales revenue, and owns $1.2 billion in distribution and energy assets, and says it delivers the nation's lowest electricity prices and most reliable network.

Despite being 50 per cent owned by ACT taxpayers, ActewAGL's corporate structure as a private venture means that it has no legislative requirement to publicly report the extent of its executives' pay.

Responding to a request from Fairfax Media for details of the remuneration packages paid to Mr Costello and his top five executives, an ActewAGL spokeswoman said the information would not be disclosed.

''ActewAGL will not be providing any details of the remuneration packages of any ActewAGL employees,'' the spokeswoman said in a statement.

''ActewAGL is very different to ACTEW Corporation - it is a private, commercial-sector business.''

Assistant secretary of the NSW branch of the Electrical Trades Union, Neville Betts, who represents more than 500 ActewAGL workers, said his union wanted the company's biggest pay deals publicly declared.

''The ETU believes that the salaries of executives should be declared publicly,'' Mr Betts said.

''There is a monster in the cupboard when it comes to salaries, that is the salaries of the CEO and executive of ActewAGL.

''The ETU have requested that the CEO and executive salaries be provided but the management have refused to provide the details.

''The ETU suspects that the obscene salaries don't end with Mr Sullivan but, in fact, is the tip of the iceberg.

''The public have a right to know the cost of these people.''

The company spokeswoman said ActewAGL had a very strong governance structure and that Mr Costello's salary was approved by the board, which includes both Mr Mackay and Mr Sullivan.

''ActewAGL is particularly proud of its strong governance structure, which includes a safety and environment committee, an audit and risk management committee and a remuneration committee,'' the spokeswoman said.

''The remuneration committee deals with an array of remuneration matters for the ActewAGL business, which includes chief executive officer and senior executive remuneration and performance appraisals.

''The salary of ActewAGL's chief executive officer is approved by the ActewAGL board.''