While the provision of pencils used to be a legal requirement, since 2020 under section 206 of the Commonwealth Electoral Act 1918 the AEC is required to provide an ‘implement or method for voters to mark their ballot papers’.
The AEC has found from experience that pencils are the most reliable implements for marking ballot papers. Pencils are practical because they don't run out and the polling staff check and sharpen pencils as necessary throughout election day. Pencils can be stored between elections and they work better in tropical areas.
There is, however nothing to prevent an elector from marking their ballot paper with a pen if they so wish.
