Seven Brits entered the singles in Melbourne, with Cameron Norrie, Kyle Edmund, Johanna Konta and Katie Boulter all falling at the first hurdle.
Dan Evans progressed before losing to Yoshihito Nishioka on Wednesday, while Heather Watson’s defeat to Elise Mertens earlier on Thursday meant Dart was the last Briton standing.
But for the world No 173 Dart it was always going to be an uphill battle against two-time Grand Slam champion Halep, and the Romanian’s quality shone through in a 6-2 6-4 win on Rod Laver Arena.
Highlights: Halep too strong for Dart
There was a flurry of early breaks as 23-year-old Dart – playing in just her fifth Grand Slam – dropped serve three times in a row to Halep, who was also broken in her first service game.
Halep was then forced to save a break point before converting her second set point after 32 minutes on court.
The second set saw Halep break twice, but the 28-year-old failed to serve it out at the first time of asking as Hart bravely rallied to stay in the match.
Halep faced another break point in her next service game, but saved it before sealing the match.
Watson: I didn't help myself - I couldn't find a rhythm
Watson crumbles against Mertens one week after beating the Belgian
British No 2 Heather Watson was knocked out of the Australian Open after 16th seed Elise Mertens recorded an impressive 6-3 6-0 victory to reach the third round.
Watson beat Mertens in three sets at the Hobart International last week, but it was the Belgian who comfortably prevailed this time around on Court 22 in Melbourne.
Highlights: Mertens crushes Watson after winning eight straight games
Watson battled to a three-set victory over Kristyna Pliskova on Wednesday, but ran out of steam when playing her second-round match a day later.
Mertens broke straight away, and though Watson hit back the Briton dropped another two times to lose the opening set.
The second set was a one-way affair, with Mertens ultimately reeling off eight games in a row in seal an emphatic victory.
Mertens’ next opponent is CiCi Bellis, who beat 20th seed Karolina Muchova 6-4 6-4.