There is no definitive answer to this question as the laws vary from state to state in Australia. In some states, such as Victoria and South Australia, it is illegal to ride a motorcycle without a helmet. However, in other states like Queensland and Western Australia, there are no specific laws prohibiting the use of helmet covers. So it really depends on where you live as to whether or not motorcycle helmet covers are legal.
If you do choose to wear a helmet cover while riding your motorcycle, it's important to make sure that the cover is properly secured so that it doesn't come off while you're on the road. Also, be sure to check the local regulations regarding motorcycle helmets before hitting the road - just to be on the safe side!