The striker cancelled out James Perch's opener in the second half before scoring the winner in extra-time at Loftus Road on Monday to set up a final against former Rangers first-team coach Steve McClaren's Derby County side on May 24.
After a goalless first leg last Friday, Perch settled Wigan nerves early on when he evaded the attention of Clint Hill to finish James McClean's cross from the left and punish QPR's slack defending after nine minutes.
Harry Redknapp's men were level courtesy of a 73rd-minute penalty, though, with Austin hauling the hosts level after Gary Caldwell had clattered into Junior Hoilett just inside the area.
Former Burnley frontman Austin was on target again six minutes into extra-time, as he fired home the winner to give QPR a chance of returning to the top flight at the first attempt.
Perch's goal was the first notable action of the game before McClean went down in the penalty area with goalkeeper Rob Green approaching, but was correctly adjudged to have dived by Mark Clattenburg.
McLean should have put the tie beyond the hosts just after the hour-mark when he was first thwarted by Green's impressive save before directing the rebound against the post.
His miss was made all the more costly when Austin tucked away the equaliser from the penalty spot 17 minutes from time following Caldwell's clumsy foul on Hoilett.
Austin was on target again six minutes into extra time when he beat Scott Carson to Bobby Zamora's lofted ball and prodded home.
Despite plenty of late pressure from Wigan, QPR stood firm to condemn former Brentford boss Uwe Rosler to play-off heartbreak for a second successive season.
