What are the qualifying criteria for registration as an overseas elector?
In addition to having your name in the Registers of Electors, :
(a) you must have resided in Singapore for an aggregate of two years during the period of five years of being away as of the cut-off date of the Registers of Electors; OR
(b) you are - (i) on full-time training or full-time service abroad as a member of the Singapore Armed Forces, or as a public officer or an employee of a Singapore public authority;
(ii) on full-time training which is sponsored by the Singapore Government or any Singapore public authority;
(iii) employed outside Singapore by an international organisation of which Singapore is a member, or any other organisation designated by the President under the Constitution of the Republic of Singapore; or
(iv) the spouse, or a parent, child or dependent of a person referred to in sub-paragraphs (i) to (iii) above.
The above are provided under section 13A(1)(a) and (b) of the PEA.
http://www.elections.gov.sg/overseasreg_faqs.htm#5WTF