Revenue is warning that those who under-declared their Local Property Tax return will be getting letters from April.
Chairwoman Josephine Feehily said they have already tackled some for lowering the value of their home, and received money from them.
Meanwhile, it has been revealed at the Public Accounts Committee that several hundred thousand, and possibly close to half a million, never paid the household charge.
There is a compliance rate of 93% on the property tax for 2013 and 1000 people a day are still paying it.
But Revenue is now going after the predecessor – the household charge – and those who did not pay it.
The two databases suggest 460,000 never paid the household charge, though Josephine Feehily admits that may be overstated because of the difference in databases.
Revenue's also now focusing on those who under declared the value of their homes - and what your neighbour did will have an impact:
"You can certainly take it that the further out people are, not just from the estimate, but from norms - we now have, because of the compliance rate, norms for areas."
She said they have already challenged a handful of people on the issue and received payment.
If homeowners believe they have under-declared the tax, they can change it online until the end of March with no interest or penalties. Further letters from Revenue will be issued in April.