The Dunnets are one of the families who've written to an ACT inquiry, calling for changes to the calculation of rates for owners of units and apartments across Canberra to be scrapped.

While rates are increasing all over the ACT as the government abolishes the "inefficient" stamp duty tax and moves towards a "more efficient" land-based method of taxation, property investors were stung twice in last year's budget, with huge increases to rates and land tax.

Before, rates were calculated after the property's unimproved value had been divided between the number of apartments of units there were on site.

That meant most apartments fell into the lowest tax bracket.

Now the rates are calculated on the entire land value, then divided between the number of units, pushing most unit owners into the top tax tier.

The change helped the ACT collect $487 million in general rates last year.

Similarly, land tax is now also calculated on the entire value of the land, rather than being divided by the number of dwellings first.

The change helped to bring in an extra $20 million this financial year.

While the government has said rates for units remain "significantly less than houses with similar market values and rental returns", those who've sunk their life savings into investment properties to secure their financial future feel differently.

Self-funded retirees Peter and Carol say they were "a bit late for the super-club" and instead invested in property for their retirement.

It took them many years and many sacrifices to pay off their three properties, but now they feel their nest egg is being "ransacked" by a government that "refuses to live within a budget".

The rates on the one-bedroom unit in Manuka they have owned for 20 years rose from $1153 in 2013-14 to $2016 in 2017-18.

Land tax jumped from $1142 to $3044 in the same period.

That means the couple, who lives off $50,000 a year, have had to "scrimp and save" to find another $90 a week to cover their costs.

"Even in our working life we weren't rich, we've always been careful with money, we haven't been splurgers," Carol said.

"We wanted to always be able to look after ourselves like we always had. We don't rely on the pension or anything like that, we're not taking from the government we're giving.

"The increases in electricity and gas as well as rates and land tax are making it very difficult for people on fixed incomes, who get no government handouts because they have lived within their means and invested wisely. We are the ones who are just above the pension line."

Son Tim Dunnet bought his first investment property at 19, after saving nearly everything he earned from his job at Woolworths.

Now 40, and without enough superannuation to retire on, his portfolio of five properties are his retirement plan.

But while his Phillip unit has lost $50,000 of value in the last decade, his costs have increased exponentially.

In 2013-14 his rates were $1182 and land tax was $491. In 2017-18 rates were $1488 and land tax was $2029.

"We've sacrificed a lot over the years, my wife, my family, to buy investment properties. We're still sacrificing now. We don't want to rely on the government when we retire," Tim said.

"I say it's fair enough that we need to pay a little more in rates but we're being gouged by the government on rates and on land tax."

Tim says the change hasn't left them in poverty, but it means they've had to rein in the family budget severely.

The occasional dinner out with his wife was the first item to be cut, as was the yearly south coast holiday. They've also decided not to enrol the children in swimming lessons next term to save another couple hundred of dollars.

He said the government appeared to think all landlords were rich and could afford to absorb the increases.

"How do they know I can afford it? They've hit us twice which isn't fair at all."