"The Coins of Malaysia" says composition is copper. According to "The Encyclopaedia of the Coins of Malaysia, Singapore and Brunei, 1400-1967" and other sources, it's bronze.
References to additional information:
[Book] Saran, Singh. The Encyclopaedia of the Coins of Malaysia, Singapore and Brunei, 1400-1967. Kuala Lumpur, Malaysia: Malaysia Numismatic Society, 1986. pp 351-372.
[Book] Kavanagh, Kevin F. 1969. The Coins of Malaysia, pp 65-73
[Book] Remick, Jerome. 1971. British Commonwealth Coins., pp 489-492