Most whistle-blowers who report tax cheats are not doing it for the money.
Eight in 10 people who tip off the Inland Revenue Authority of Singapore (Iras) do not want a reward, now worth up to $100,000.
Since 1992, when the cash reward for whistle-blowers was introduced, only three have accepted $10,000 each.
Miss Loh Lee Kim, Iras assistant commissioner of investigation and forensics, told The Sunday Times: "We think most informants don't want the reward because they feel that something wrong is happening and they just want the right thing to be done."
But some may be out for revenge against the tax cheat for some personal reason, she said. Tip-offs come from business rivals, employees, family members, neighbours and cheated spouses, among others.
Iras receives about 900 tip-offs each year, looks into all of them, but investigates only about 5 per cent as the bulk turn out to be unfounded allegations.
"It's very common for people to tell us, 'Mr A is very rich, I think he didn't declare his taxes, go and check on him'," said Miss Loh. "Of course, we will do our due diligence, but what we want is a very specific description of the modus operandi and, where possible, documentary evidence."
The whistle-blower reward used to be 10 per cent of the tax recovered subject to a maximum payout of $10,000. In 2010, the reward was raised to 15 per cent of the tax recovered, or up to $100,000.
Nobody has received the bigger payout, but Miss Loh expects a few whistle-blowers to be offered about $100,000 each soon. She declined to say more.
She said Iras guards the identity of informants closely and only a few officers handling a case know who they are. Whistle-blowers are paid a reward only if the information they provide leads to tax being recovered.
One whistle-blower who did not ask for monetary compensation alerted Iras to a Filipino salesman working in a jewellery shop.
He was the mastermind of a syndicate targeting the Electronic Tourist Refund Scheme that lets tourists claim a refund of GST paid for purchases.
He printed receipts from genuine jewellery sales to forge refund tickets, then got his Filipino accomplices to use them to claim the GST refund.
The scam went on for about a year and the salesman made at least $14,000 before he was caught. He was jailed for a year in 2012 and had to pay a penalty of about $43,000 - three times the sum cheated. One accomplice was also fined.
The whistle-blower could have received a reward of close to $2,500 if he or she had wanted it.
Dr Ernest Kan, chairman of the Singapore Institute of Accredited Tax Professionals, said the effectiveness of the whistle-blowing programme lies in getting people to report suspected abuse, but awareness remains generally low.
Whistle-blowers can e-mail email@example.com or write to Inland Revenue Authority of Singapore, Investigation & Forensics Division, 55 Newton Road, Revenue House, Singapore 307987.
To inform the taxman of your tax errors or omissions made in the past, e-mail iit_compliance@