As a Singapore citizen or tax resident in Singapore, the income from your employment exercised outside Singapore on behalf of Singapore government is deemed to have been derived from Singapore.
All your gains from such employment (including overseas allowances) are taxable in Singapore.
Your employer will send your employment income details (including all allowances paid to you while you are working outside Singapore) to IRAS electronically.
The tax on overseas allowances will be remitted if applications for tax remission are approved by Ministry of Finance (MOF).