The only developers who have been asked to pay are the government's own Land Development Agency, in work at the former Downer school, and the federal government's National Capital Authority during work on Constitution Avenue.

Icon Water wants to impose a new charge on all developments – from dual occupancies, to offices, shops, hotels and apartment buildings – to raise about $90 million to pay for upgrades over 20 years. Icon will pay the same, for a total of about $180 million.

The charge would apply to only four areas – Belconnen, Fyshwick, north Canberra and Woden.

The inner south, Tuggeranong, Weston Creek, the Molonglo Valley and most of Gungahlin are exempt because no population-related upgrades are needed in those areas.

Development will be most expensive in Woden, at a proposed $3000 for each extra person.

But the biggest spend is in north Canberra, where Icon expects to spend $102 million upgrading infrastructure, to accommodate 28,200 extra people over 20 years. Because the work is spread over so many people, the cost per person is less.

Icon says the charge will "increase clarity and fairness" and bring Canberra into line with elsewhere.

"Right now, a developer may be left as the last person standing and have to foot a bill for an entire suburb," Icon says.

Asked initially for detail on how many times developers had been charged for infrastructure, Icon Water first said it could not disclose the information because of privacy. But pushed, it cited only two cases – both paid by governments.

"Although only two augmentations have been triggered in recent years, Icon Water is proactively introducing this new code to ensure clear and simple arrangements are in place to support the growth and renewal of Canberra," a spokesperson said. "Going forward we expect that as brownfield developments increase, further capacity upgrades would be triggered at a much higher rate than in the past. This is the key reason for proposing to introduce a scheme now, to ensure development is sustainable."

Icon Water has now extended its call for feedback to March 17. It had planned to impose the charge on July and apply it to developments approved after March 31 this year. It is now considering a longer phase-in to exclude some developments already in train, but but for every development that escapes the levy, the amount per person will be increased to compensate.

The levy as it stands, without a longer phase-in, is proposed at:

The Belconnen upgrades are in the town centre and the new suburb of Lawson. The Fyshwick upgrades are in the East Lake area between Kingston and Fyshwick, to be opened to residential development. The spend for Woden is around the town centre. And the north Canberra spend is in the Northbourne corridor.

Icon Water has based its levy boundaries on sewerage catchments and some of the boundaries cut through suburbs. The result is that developments in Deakin will face the Woden charge if they're west and south of Kent and Strickland Crescent but enjoy the south Canberra exemption elsewhere.

In the north, Icon's boundaries include Mitchell in the Canberra north zone, along with parts of Franklin and Mitchell, making some parts of those suburbs exempt from the charge and some not.

The population numbers don't correlate precisely with numbers of people – since for Icon's calculation, a new apartment is worth two people, a free-standing house 3.6 people, each worker in a new shop or office as 0.3 of a person, each restaurant seat as 0.1 people, and so on.

The cost is only for extra people, so a person demolishing and rebuilding their home would not pay. But a person replacing a house with two townhouses would pay for 1.4 extra people, equating to $4200 in Woden, $2520 in north Canberra, and $1960 in Belconnen (the formula counts a house as 3.6 people and the townhouses as 2.5 each).