Middlesex captain Peter Handscomb is to leave the club following the County Championship game with Durham.
Handscomb, 31, who has overseen an unbeaten start to the season, is to join the Australia A side for their series with Sri Lanka A before returning home for family reasons.
Tim Murtagh will take over as skipper of the red ball and One-Day Cup sides.
The county have also signed Australian pair Jason Behrendorff and Chris Green for this season’s T20 Blast.
Left-arm paceman Behrendorff, 32, will arrive at Lord’s once his commitments with Royal Challengers Bangalore end.
He replaces fast bowler Shaheen Shah Afridi who will not be returning to the club after Pakistan’s series against West Indies.
Green, 28, played for Middlesex last season and will come in for Afghanistan spinner Mujeeb Ur Rahman in early June.