The auditor-general has told the Revenue Department to collect more taxes from some 60 politicians in the previous two governments because what they paid did not match the increases in their declared wealth.
Pisit Leelavachiropas said on Friday that the move was part of his policy to make tax collections fairer.
"As we're undergoing political reform, we found several politicians were substantially richer but the taxes they paid remained the same," he said. "When I asked the Revenue Department whether they paid taxes on the increased amounts, they told me these politicians were not unusually rich.
"But we see things differently. Even though they're not flagged, they need to pay taxes on the additional income."
The process of collecting the extra tax has to be started quickly as the window for a tax reassessment is five years, he explained.
Mr Pisit stressed he was not targeting anyone in particular, noting that the investigation was not limited to those in the Democrat or Pheu Thai parties but also covered people in other parties in the two governments.
The new move follows the successful push by Mr Pisit's office for a review of the tax case against former premier Thaksin Shinawatra and his children despite the Revenue Department's claims that it could not be done based on existing rules.
Mr Pisit did not elaborate on the amount of additional tax he believed might be due from the politicians, the additional income involved or higher valuations of additional declared assets. If the latter is the case, tax can only be collected when the assets are sold, according to existing rules.