Why did Jay Ryan quit Sea Patrol after season 3?

Billy Webb (Spider) leaves Sea Patrol because he got a new posting on another warship.

Spider is last seen in Sea Patrol in the last episode of season 3.

Along with 2 other actors/actress (Nav & Buffer) leave Sea Patrol at the end of season 3.

I don't think that Sea Patrol will do very well after the loss of 3 Excellent actors.