Despite the name, it has nothing to do with the Indonesian city — 'bandung' here is old Malay for 'paired,' referring to the rose syrup and milk mixed together.
Air bandung is a bright pink drink made by stirring rose syrup into evaporated or condensed milk and pouring it over ice. In Brunei it's a fixture of iftar tables during Ramadan and Eid, sold by home cooks and roadside stalls alike.