Cronulla have won six straight matches for the first time since 2012 and moved top of the NRL ladder courtesy of a 30-28 win over Brisbane.
The Sharks led 28-6 at the break after dominating the first half.
But the Broncos came storming back after the break to draw within two points before the home side held out for a deserved win at Shark Park on Sunday before 14,406 fans.
Led ably by skipper Paul Gallen and well supported by James Maloney and Chad Townsend, Cronulla join the Broncos, North Queensland and Melbourne atop the NRL ladder after nine rounds.
The loss was the Broncos' first in seven games.
After an early Corey Parker penalty goal for offside the Sharks dominated most of the first stanza.
Sosaia Feki crossed in the 12th minute courtesy of a bullet-like pass from Ben Barba.
The home side went further ahead when Paul Gallen pounced on a Michael Ennis kick in the 16th minute for a lead of 10-2.
The Broncos hit back straight away through Jordan Kahu after Josh Townsend knocked on from the restart. But that only served to inspire the Sharks further.
James Maloney crawled his way over for the Sharks' third in the 26th minute before Feki added his second on the half hour.
Townsend did well to pick up and ground a loose Maloney kick just before the break to score under the posts, putting Cronulla out to a 28-6 lead at the break.
However the Broncos dominated after that through second half tries to Corey Oates, James Roberts and Sam Thaiday, owning the ruck as the Sharks had in the first half.
Cronulla held firm in the dying minutes as the NRL breaks for the representative round.
Feki didn't come out for the second half due to a hamstring issue.
Broncos forward Alex Glenn suffered a calf injury in the 45th minute and didn't return.
Gallen could be in judiciary trouble for a first half incident on Josh McGuire.