Property website Daft.ie has criticised the Revenue Commissioners' online guide to the new property tax.
The guide, which helps homeowners estimate what tax band their property falls into, was launched by Revenue yesterday.
The Revenue insists figures supplied on the website are only estimates, but there is concern about the accuracy of the guide.
People will begin to receive letters detailing the amount of property tax owed this week.
Property economist with Daft.ie Ronan Lyons says the value is only based on property type, which presents a number of problems.
"They don't have key information, for example the number of bedrooms for every property in the country or square meterage.
"That presents a number of issues not just in Dublin where values can vary hugely in a short distance, but also in other parts of the country," he said.
It is estimated it could take up to four weeks for all homeowners to receive the details of the amount of tax owed.
Dr Keith Walsh from the Revenue Commissioners said its online guide should not be relied upon in isolation.
Speaking on RTÉ's Today with Pat Kenny, Dr Walsh said Revenue has established what he called a "reasonably good example of the Irish property market at the current time".
However, he said this is a self-evaluation tax, and that the responsibility for supplying an accurate house evaluation ultimately lay with them.
He said: "I don't think we'd be saying people should be looking at our valuation guidance in isolation, and just take whatever is given there."
He said people should look at Revenue's valuation guidance, which gives an average for an area, along with the property price register and what estate agencies are quoting, to build up the best picture of what properties are worth.
Revenue said there were 207,000 hits on the property valuation website yesterday, but these were not necessarily unique users.