Icon Water made about $76 million in dividends for the ACT Government in the last financial year, and the June budget laid out a whopping $33 million boost over the next three years – rising to $104.8 million by 2019-20.

But that forecast, an Icon spokeswoman has confirmed, relies on an assumption the Independent Competition and Regulatory Commission will give the water supplier a "4.5 per cent real price increase" in 2018-19, as well as increasing revenue from the ActewAGL joint venture in 2019-20.

The assumption was made despite a commission decision on a new water tariff structure still months away and that the tariff review was not meant to raise any extra revenue for Icon.

Former ACT Treasury official, now adjunct professor at University of Canberra, Dr Khalid Ahmed said the assumption had broken a basic rule of budget forecasting – that a government agency should not pre-empt the outcome of an independent regulator's decision.

"They are assuming that they will get a 4.5 per cent real price increase, which means once inflation is taken into account it could be closer to a 7 per cent price increase, which will drive up their revenue and the dividend," he said.

Dr Ahmed also compared the past five years of Icon's actual dividend results against the original budget forecasts.

He said the analysis showed the forecasts for Icon's dividend to the ACT government had been wrong by up to 16 per cent on average over the period 2011-12 to 2015-16.

"The budget forecasts for the dividend were in error – lower or higher – by 16 per cent per annum on average," Dr Ahmed said.

"There is a bias towards overestimating Icon dividend revenues in the budget, with actual results coming approximately 7 per cent below budget on average."

When compared to actual results, the budget estimates for Icon Water's dividend have been: 20.3 per cent higher than the result in 2011-12, 7 per cent higher in 2012-13, 29.5 per cent higher in 2013-14, 13.7 per cent lower in 2014-15 and were estimated to be 9.2 per cent lower in 2015-16.

The Icon spokeswoman said the forecast price increase was included "to reflect that the ICRC will forecast water consumption closer to what is currently being experienced", about 42GL a year, compared to an earlier industry panel forecasts of 44GL a year, which had not been reached.

But she also said "the level of [water] prices was uncertain beyond 2018", seemingly contradicting Icon's own assumptions underlying the dividend forecast.

The corporation confirmed the water price rise was the main contributor to a forecast $16 million rise in dividends from 2017-18 to 2018-19 and the joint venture investment, ActewAGL, was the main cause of the further $14 million rise in dividends forecast from 2018-19 to 2019-20.

Dr Ahmed said: "The reality is, if these forecasts are to be believed, that the poor taxpayers if they consume less, they'll pay more, but if they consume more, they'll pay more too."

The Icon spokeswoman said the corporation also used the "latest estimates on customer numbers, water consumption, energy investment results and operating expenses to determine the profit and dividend forecasts".

It remains unclear why Icon included a 4.5 per cent real price increase in 2018-19, given the water supplier also acknowledged prices were "uncertain" from June 2018 due to the ICRC tariff review.