If you discover a mistake in a return you have already filed – whether it understated or overstated your income – the right course is to correct it promptly rather than wait for IRAS to find it.
For genuine errors, you can revise your filing or lodge an objection to the assessment within the stipulated period. Where the error resulted in tax being underpaid, IRAS's Voluntary Disclosure Programme (VDP) allows you to come forward voluntarily; timely disclosures are treated far more leniently, typically with substantially reduced or waived penalties, compared with errors uncovered by an IRAS audit.
Act quickly, keep documentation explaining how the error arose and how you corrected it, and pay any additional tax as soon as possible to limit late-payment penalties. Deliberate under-declaration is treated seriously and can attract heavy penalties, so honest, prompt disclosure is always the better route.
Apexia can help you assess the error, prepare a voluntary disclosure or objection, and liaise with IRAS on your behalf.