According to TV Week, Coleby, 29, will play Dylan 'Dutchy' Muholland.
In August, producers announced that Jeremy Lindsay Taylor (Buffer), Saskia Burmeister (Nav) and Jay Ryan (Spider) would not be returning for the naval series' fourth season.
Meanwhile, former Neighbours actress Danielle Horvat (Taylah Jordan) will also join the series as Jessica Bird.
Coleby and Horvat will be joined by former All Saints actress Tammy MacIntosh as Commander Maxine White.
