For the second week in succession, a goalkicker had the last-minute chance to win for their team. Just like Australia's Kurtley Beale in the first test in Brisbane, Halfpenny didn't make the slightly longer distance.
In a tension-filled finish, Adam Ashley-Cooper dived over for the only try of the match in the 76th minute, five minutes after the Australians turned down a shot at goal in favour of a scrum deep close to the line, and Christian Leali'ifano converted from near the left sideline to give the Wallabies a one-point lead.
As the match drifted into stoppage time, the Wallabies conceded a penalty on half way and Halfpenny, who had only missed three shots at goal all tour, had the chance to secure the Lions' first series win since 1997. His miss gives Australia back the momentum.
"He realizes the significance of that kick at the end and he's just disappointed he missed," Lions coach Warren Gatland said. "I've seen him kick them from there before.
"Just, you know, a chance to be a hero in that moment ... unfortunately he hasn't hit it quite as well as he's capable of doing."
Wallabies captain James Horwill's euphoric and unguarded celebration followed a tumultuous week that started with him being cited for stomping in the 23-21 loss in Brisbane, being cleared after a long hearing and then discovering the International Rugby Board had launched an unprecedented appeal against the finding.
"It's all bets are off. It's square," said Horwill, who could be ruled out of the deciding match depending on the outcome of the IRB's appeal hearing on Monday. "It's sort of, you don't worry about the last two, it's now one game to win it. It's like a grand final really — you don't worry about what's happened before.
"We certainly made it hard on ourselves at times, but it saves the series for Sydney."
Lions captain Sam Warburton was inspirational at the breakdown, earning repeated turnovers before injuring his left hamstring and leaving the field in the 67th minute. He said he'd have to wait until Monday before a proper assessment on the injury, but is desperate to play next week.
The Lions won the 2001 series opener against Australia and led at halftime in the second test, but Australia rebounded to level the series in Melbourne and won with the momentum in the third test in Sydney.
After letting another lead slip, Warburton predicted the series would come down to one or two points next week as well. He said nobody could blame Halfpenny for missing the 50-meter, high-pressure kick.
"We obviously didn't finish the way we wanted but we've got every chance. We're in the same situation now that we were this morning — we've got a test series up for grabs," he said. "Obviously the players will be disappointed now for a while (but) tomorrow morning when we wake up, we've still every chance of winning this test series.
"It's not doom-and-gloom at all."
After a kicking duel between Halfpenny, who landed five penalties, and Leali'ifano in the first 75 minutes of a match tarnished by handling errors and scrum penalties, Wallabies flyhalf James O'Connor gave Ashley-Cooper a flat ball close to the line and the veteran centre crashed over.
Gatland had correctly predicted before the match that referee Craig Joubert would have "zero tolerance" at the scrum in Melbourne, with Lions prop Mako Vunipola penalized twice in the first half for collapsing and the Australian props each penalized for the same infringement before the break.
The Lions were dominant at the breakdown early, turning over Australian ruck ball three times in the opening 11 minutes and winning penalties from driving mauls.
Both teams tried to be adventurous at times, with the Lions trying an 11-man lineout near the Australian line and twice winning penalties from the collapsed rolling mauls before Halfpenny kicked his first goal in the 10th minute.
The Wallabies twice tried running the ball from their own tryline before resorting to clearing kicks. But the ball rarely reached the wings until the Wallabies started spreading it more relentlessly in the last half hour.
Leali'ifano, who only lasted 52 seconds in his test debut last week before being carried off with a head injury, scored his first points in international rugby in the 16th minute. He added two more goals to give the Lions a 12-9 lead at the break.
It took 24 minutes before anyone scored in the second half, with Halfpenny slotting a long-range penalty to extend the Lions' cushion to six points.
The huge contingent of Lions travelling supporters started singing 'Swing Low' in a bid to rouse the tiring Lions as the error-prone Wallabies poured on the attack.
After seemingly striking the killer blow with the try, Horwill said the Australians could only blame themselves for getting within seconds of surrendering the series.
"He's hardly missed a kick all tour," he said of Halfpenny's usually radar-like accuracy. "Your pulse rate certainly goes up and we were disappointed. We let it get to that stage where we allowed them to have a shot.
"We should have been able to run the clock down a bit more calmly than having a kick right on the death for them to win it."