http://www.elections.gov.sg/voters_compulsory.html
What would be regarded as a valid and sufficient reason
for not voting?
The following explanations have been accepted as valid and
sufficient reason for not voting :
You need to substantiate your explanation when applying to
restore your name to the register of electors. For example, for
those who claim to be overseas to work or study, or to be living
with a spouse who is working or studying overseas, the passport
plus an employer's letter or educational institution's letter, and
marriage certificate would be required. For those who were on an
overseas vacation or business trip, the passport plus documents
showing that the trip had been planned before Nomination Day should
be produced, such as receipts from travel agencies, air tickets,
hotel booking receipts, or a suitable letter from your employer.
Those who did not vote for health reasons should produce a medical
certificate, hospital bill or the child's birth certificate.