Mix oats, coconut, flour and sugar in a large bowl.
Butter 2 baking trays and heat oven to 150°C (300°F). Heat butter, water and golden syrup in a saucepan, stirring, until butter melts. Remove from heat and quickly stir in bicarbonate of soda until light and frothy.
Pour the mixture into the dry ingredients, mixing quickly and thoroughly. Roll mixture into balls the size of walnuts and place on trays, allowing room for spreading.
Press down gently with a spatula, and bake for 20 minutes until lightly golden and still slightly soft. Remove trays from oven and leave biscuits to cool on the trays for 10 minutes. Transfer to wire racks to cool completely until crisp on the outside, but still a little chewy inside.
Store in an airtight jar as soon as biscuits have cooled.