The 23-year-old reported for training with the squad on Monday, the Welsh Rugby Union announced.
King, who can also play in the back row, made his Test debut during Wales' tour of Japan in June.
Wales' second-row resources have been affected by the unavailability of Bradley Davies and Luke Charteris.
Davies, who started the autumn series openers against South Africa and Argentina, suffered a shoulder injury during the Pumas game and subsequently underwent surgery.
Charteris, meanwhile, has returned to his club Perpignan, who are not obliged to release him for the Australia game as it falls outside the International Rugby Board's autumn Test window.
Newport Gwent Dragons lock Andrew Coombs, who had been due to pack down at blindside flanker against Tonga last week, withdrew from that game due to a back injury.
Wales have lost their last eight Tests against Australia, although half of those defeats were by three points or less.
Wales coach Warren Gatland is due to announce his team to face the Wallabies on Thursday.
- Sports & Recreation
- Luke Charteris
- Bradley Davies