Add the sugar and 500ml (18fl oz/2¼ cups) water to a medium saucepan. Place over a low-medium heat for 5 minutes, or until the sugar has dissolved. Increase the heat to medium and simmer for 5 minutes.
Remove from the heat and add the hibiscus petals. Cover and allow to cool and infuse, then strain the syrup and discard the hibiscus petals.