Below taken from the Customs website explains clearer on the GST relief for travellers bringng goods in.
Period away from Singapore
Value of Goods
48 hours or more
S$600
Less than 48 hours
S$150
Such goods include new articles, souvenirs, gifts and food preparation which are for your personal use and not meant for sale.
Holders of a work permit, employment pass, student pass, dependent pass or long-term pass issued by the Singapore Government and crew members are NOT eligible for GST relief.
You have to pay the taxes on goods exceeding your GST relief and on goods you are carrying for or on behalf of other persons. GST is levied on the value of the goods, which may include the cost, insurance and freight (CIF) plus other chargeable costs and the duty payable (if applicable).
It is your responsibility to make an accurate and complete declaration of all goods exceeding your duty-free concession and GST relief in your possession, including goods you are carrying for or on behalf of other persons.