The new system of rates should include a cap for those hit with bills they cannot afford to pay, former Stormont finance minister Mark Durkan says.
Many householders in NI face an increase in their rates
Current minister David Hanson insists that introducing a cap would benefit a tiny percentage of people at the expense of the majority of ratepayers.
However, the SDLP leader said a cap would make the new system fairer.
"No tax can be fair if it's going to force people from their lifelong homes - it is simply wrong," he said.
"It is to try to protect people from hardship - the fact is the measures that David Hanson is imposing are going to put people into real hardship, it's going to force people to move from their homes," he told Radio Ulster's Inside Politics.
"It is a cruel tax which cannot be justified, and if a cap is a way of stopping that, fair enough."
Mark Durkan said the new rates system is a cruel tax
Under the new proposals, rates bills will be calculated according to the property's value in January 2005.
Mr Hanson has insisted he is determined there will be no cap on bills, but said his decision could be overruled if devolution returned by the November deadline.
The first rates valuations were sent to householders in Northern Ireland at the start of July, and those who wish to challenge or query their rates estimate have six months to do so.
More than 7,000 homeowners have already challenged the rateable estimates of their homes.
Not everyone will have to pay all or any of the new charges. More than 175,000 households will pay a reduced rate or no rates at all.
Anyone who wants to check the value of their home can do so by logging onto the Valuation and Lands Agency's website at www.mycapitalvalueni.gov.uk.