Ivan Toney's 24th league goal of the season put the Bees ahead on the half-hour mark.
But on-loan West Brom duo Austin and Sam Field turned the game on its head by scoring in the space of four second-half minutes.
It condemned Brentford to a second successive loss – three days after a defeat at home to Barnsley ended a 21-match unbeaten run in the Sky Bet Championship.
Field, on as a substitute for his R's debut, thumped in a left-footed volley from 10 yards out after Brentford failed to clear Todd Kane's 72nd-minute cross from the right.
Kane was also involved in the build-up to the second Rangers goal.
His cross-field ball was pounced on by Lee Wallace, who pulled back for Austin to fire home via a deflection
Brentford had been the better side and went ahead when Mattias Jensen whipped in a free-kick from the left and Toney's slight touch at the far post nudged the ball beyond keeper Seny Dieng and into the far corner of the net.
The visitors continued to dominate after taking the lead, with Toney's deflected free-kick flashing just over and Dieng pushing away Jensen's long-range strike.
QPR improved after the interval and went close to equalising when on-loan Fulham midfielder Stefan Johansen curled a free-kick narrowly over.
But Brentford looked fairly comfortable before being emphatically punished for two shoddy defensive lapses.
And they ran out of ideas in the attacking third in the final stages, with Toney's header over from Jensen's corner the closest they came to an equaliser.
Thomas Frank's side remain second in the table but his team's recent wobble will no doubt be of concern to the Dane.
On the other hand, Rangers' former Brentford boss Mark Warburton has seen his team recover well from a recent slump.
Austin's return to his former club has had the desired effect and Rangers have lost just one of their past eight matches – a run which has included five wins.