The Championship club crashed out to the League One outfit on penalties, as Lee Johnson's side booked their place in the quarter-final.
The game finished 0-0 in normal time, but Austin thought he had headed home the winner from Albert Adomah's deflected shot.
The goal was controversially ruled out for offside, with replays showing the striker clearly onside.
Austin accepted that mistakes happen, but said the decision would be quite costly for the club.
"When I headed it in I had a split look at the linesman and he hasn't put his flag up straight away, waited four or five seconds and wasn't until I was celebrating he put his flag up," Austin told talkSPORT Breakfast.
"It was an appalling decision, you could hear the frustration in the manager's voice after."
When asked if the officials apologised after the game, Austin said: "Nah… I can't repeat what I said to the lino. You know what they're like they just said 'it was offside'.
"It was human error, but has some serious consequences for the football club."
The decision highlights the fact that VAR is not in use in the competition at this stage, in previous years it has been used for the semi-finals and finals.
"It's either we have it in the competition or we don't," Austin added. "You can't just have it in the Premier League games, if we are going to use it we have to use it for the whole competition.
"Unfortunately it was one of those things, it was human error and I get it, but it's a hell of an error to get wrong.
"I don't really understand what the thought process is. The goalie is there and the defender is on the line.
"I wasn't very happy. I'm disappointed we didn't win, but also on a personal note for the goal you think 'happy days, six yards out open goal, perfect'.
"I guess personally I'm not getting the rub of the green."
QPR boss Mark Warburton was also left stunned by the decision and had to be careful with his words.
"I've just seen it. What can I say without getting fined?" Warburton said.
"It's staggering to be that wrong on a major decision in a game of that importance. I was told it was immediately offside by the fourth official.
"Having just seen the picture I've just seen, wow! It's an appalling decision which has cost the club in all sorts of ways.
"The picture I've just seen, we're not talking about an inch or a marginal decision. It's feet, between Charlie being onside and offside. Those types of decisions, you cannot get it wrong.
"With what's at stake, the financial implications for the club, big crowds home and away, everything which comes with progression to the last eight, I've never been so shocked to see a picture like that.
"To be that far onside, you have to question what did they see? I hope this doesn't get me too much of a fine."