The boys will compete in Melbourne while the girls will be in Hobart from October 2-7.
The Torpedoes, Claremont Dolphins and Melville will represent WA at the championships.
Pethick, a Scotch College student from City Beach, has been training six times a week to prepare for the competition where selectors will choose the Australian born 1996 team.
The teenager is part of the Australian squad after making the Torpedoes team for the second year.
Hughes, of Mt Claremont, has also been playing with the UWA club for two years and is part of the girls’ Australian born 1996 squad.
The Methodist Ladies’ College student is a driver for the Torpedoes and started playing water polo nine years ago.