The High Court has ruled that Howard-era laws that close the electoral rolls on the day that writs for an election are issued are invalid.

The Australian Electoral Commission will now scramble to contact an estimated 100,000 Australians to inform them they are entitled to vote at the election on August 21.

Activist group GetUp! brought the constitutional challenge, arguing the laws effectively disenfranchised young people who were prevented from registering when the rolls closed on July 19.