Vision. You MUST be able to read a vehicle number plate, in good daylight, from a distance of 20 metres (or 20.5 metres where the old style number plate is used). If you need to wear glasses (or contact lenses) to do this, you MUST wear them at all times while driving. The police have the power to require a driver to undertake an eyesight test.
Laws
Section: Rules for drivers and motorcyclists (rules 89 to 102)
Subsection: Fitness to drive (rules 90 to 94)
Subsection: Fitness to drive (rules 90 to 94)