As useful as a wart on the hip

absolutely useless

same as:
As useless as a chocolate fireguard
As useless as a glass door on a dunny
As useless as a soft cock in a nurse's dorm
As useless as an arse pocket in a singlet
Useless as an ashtray on a motorbike
Useless as the bottom half of a mermaid
Useless as tits on a tomcat