According to TV Tonight, Jeremy Lindsay Taylor (Buffer), Saskia Burmeister (Nav) and Jay Ryan (Spider) will not be returning for the next season of the naval drama.
Producers Hal and Di McElroy told Sea-patrol.com: "Actors doing TV series are usually contracted for three series, so departures are always on the cards. Of course in Navy, postings are rarely longer than two years, so there are always regular crew turnover anyway."
They added: "We'll miss them. All three are fine actors and we've become good friends."
Season four will reportedly comprise 16 episodes rather than the usual 12.