Ralston suffered a stress fracture of the lower back and has been replaced with Hardie.
The ICC has confirmed that the Event Technical Committee of the ICC U19 Cricket World Cup 2017 has approved Aaron Hardie as a replacement player for Jason Ralston in the Australia squad for the tournament.
Ralston suffered a stress fracture of the lower back and has been replaced with Hardie, who like Ralston, is also a right arm fast bowler and right-handed batsman.
Ralston had returned astounding figures of 7/15 in a league game against Papua New Guinea, which was a tournament record before his compatriot Lloyd Pope bettered it with eight for 35 against England in the quarter-finals.
The replacement of a player requires the approval of the Event Technical Committee before the replacement player can be officially added to the squad.
The Event Technical Committee of the ICC U19 Cricket World Cup 2018 consists of Geoff Allardice (ICC, ETC chairman), Chris Tetley (ICC representative), Brendan Bourke (Host Tournament Director), Catherine Campbell (Host Nominee), Tom Moody and Ian Bishop (both independent nominees).