Social advocacy groups are warning homeowners on a fixed income are among those hardest hit by the rates rises, after the ACT government changed the formula for calculating rates on apartments.

Before, most apartment owners fell into the lowest marginal rates bracket, because the value of their land was divided into the number of apartments before rates were calculated.

Now, rates are worked out on the value of the entire block before the rates bill is divided among apartments, pushing apartment owners into the top marginal rates bracket. The same change was made for land tax.

Unit owners still pay on average 40 per cent less in rates than those in freestanding houses, and will get a $100 rebate this year as a transition measure.

But already, 23 applications for deferral on the grounds of hardship have been filed, compared to nine at the same time last year.

Council of the Ageing ACT executive director Jenny Mobbs said the rates increase had prompted some elderly Canberrans to reconsider downsizing from their family home.

"People are quite concerned about what's the right thing to do - stay in the family home which is too large for them or downsize and get hit with rate rises," Ms Mobbs said.

"This has come on top of people getting [more expensive] energy bills too. It's quite severe, particularly for those on pensions."

ACT Council of Social Services director Susan Helyer said more help needed to be given to those on the margins.

"We support the move towards a land tax revenue base for the ACT but recognise if there are some people put in significant hardship, like those on a fixed income whose property prices are increasing significantly but their income isn't, we think that they should be assisted through concessions. If it puts people at risk of losing housing, it's not outcome we should be looking for," Ms Helyer said.

St Vincents De Paul national manager of policy and research Gavin Dufty said the rates rises and utility fee hikes were a "perfect storm".

"What you've got is clustering of charge increases and for low-income homeowners it's an incredibly stressful time. The big challenge is proud pensioners, they are the least likely to seek support," Mr Dufty said.

An ACT government spokeswoman admitted no modelling had been done about the impact of rate rises on homeowners with a fixed income.

"However, information on households that qualify for rates concessions indicate that pensioners and those on fixed incomes can live in properties with a range of different values," she said.

She said eligible pensioners received a "substantial rebate" on the rates for their primary home, which reduced their rates by 50 per cent up to a maximum of $700.

Isaacs retiree Dianne Peacock owns one townhouse in a complex of 22, and says the prospect of finding another $614 in her budget for rates fills her with "horror".

Last year her rates bill was $1698, but this year it is $2312.

"I don't know how I will manage," Ms Peacock said.

While young renters unable to get a foothold in the property market are often the face of Canberra's housing affordability crisis, Ms Peacock said older people are also suffering.

"Housing affordability also affects older people. We wonder 'how am I going to stay in my home'," Ms Peacock said.

"A dilemma for me is I'm very supportive of what the government is doing, I'm supportive of the move to light rail and the modernisation of Canberra.

"My view is the government hasn't fully realised the impact on people and they need to have another look at that. They need to think about the cost to their supporters."

Ms Mobbs said older Canberrans needed to go into any new property purchase "with their eyes open".

"People on the pension or using super have a fixed income, there's no way to change that. We are living longer, so it might have to last them for another 20 years," Ms Mobbs said.

"People need to ask the questions, what are the rates, what will they be next year. People need to start asking questions and finding out what concessions there are."

- with Daniel Burdon and Kirsten Lawson