The following from PWC Thailand tax booklet for personal tax.
"Penalties and surcharge
A taxpayer who is assessed additional tax by an assessment officer
on the grounds that an inaccurate return was filed, or who failed to
file a return, is subject to a penalty. The rate of penalty is 100% for an
inaccurate return and 200% for failure to file a return. Penalties may
be reduced by 50% if the taxpayer submits a request in writing and
the assessment officer is of the opinion that the taxpayer did not
intend to evade tax and cooperated with the officer during the tax
audit.
Any person who fails to pay or remit tax within the specified time is
liable to pay a surcharge of 1.5% per month, or fraction thereof, of the
amount of tax to be paid or remitted subject to a maximum equal to
the amount of tax to be paid or remitted.
Tax investigation and assessment
The Revenue Department is empowered to demand documents and
records for inspection for a period of two years. The period will be
extended to five years if it is found, or there is a reason to believe,
that there was tax evasion, or where a tax audit is conducted for the
purpose of paying a tax refund.
Nevertheless, under the Civil and Commercial Code, the Revenue
Department can assess tax for up to ten years."
I think it is this section from the Civil and Commercial Code
Section 448. Statute of Limitations
The claim for damages arising from wrongful act is barred by prescription after one year from the day when the wrongful act and the person bound to make compensation became known to the injured person, or ten years from the day when the wrongful act was committed.
However if the damages are claimed on account of an act punishable under the criminal law for which a longer prescription is provided such longer prescription shall apply.
Prescription
Section 193/31. Taxes 10 Years
The period of prescription for claims of the Government for taxes and rates is ten years. As to other claims of the Government relating to obligations, the provisions of this title shall apply.