When Sharks hooker Blayke Brailey darted over from dummy half to secure a 28-12 lead in the 56th minute, the Broncos appeared cooked in front of 39,042 fans at Suncorp Stadium.
But the home side scored four tries in 18 minutes, with Walsh setting up two, to usurp the Sharks in fifth position.
"I guess we expect Reece to come up with those things. He just keeps competing," Broncos coach Michael Maguire said.
"The captain (Adam Reynolds) spoke to them at halftime and sharpened them up a bit.
"It was pleasing that with their backs against the wall ... to fight the way they did and get the win at the end."
Sharks forward Teig Wilton went to hospital in the first half after suffering blurred vision from an eye injury, before winger Sione Katoa had a brain explosion on the cusp of halftime. He was sin-binned for a ridiculous touch on Ezra Mam when he was laying in the ruck.
The Broncos were too daft to make the most of it. Instead Brailey darted over from close range and the result appeared secured, but the Sharks then fell apart.
Winger Ronaldo Mulitalo was ruled to have knocked on and Walsh made the Sharks pay with a bullet pass to centre Gehamat Shibasaki.
Sharks five-eighth Braydon Trindall kicked out on the full and Broncos winger Josiah Karapani stormed in despite Sharks half Nicho Hynes falling over and claiming an obstruction.
Skipper Adam Reynolds then darted and weaved in traffic and threw a basketball-style pass for Karapani to complete his double in as many weeks. Reynolds landed the conversion from the sideline for a 30-28 lead.
When Walsh carved through Hynes to set up Shibasaki for his second, it was all over.
"Our discipline to stick to our plan and defence has been poor for weeks. Even when we were leading, you could see signs we were hanging on," Cronulla coach Craig Fitzgibbon said.
"I'm not going to sit here and make excuses. We need to do something about it. We've got to address our discipline."
The first half was a bizarre affair, with the Sharks leading 22-12 at the break.
Brisbane's kicking game and line speed were poor, with fullback Walsh giving up two seven-tackle sets.
The Sharks attacked with freedom, with halves Trindall and Hynes playing lead roles.
Trindall laid on the pass for centre Siosifa Talakai to score. He was at it again to put Wilton through a gaping hole, and fullback William Kennedy finished it off for a 10-0 lead.
The Sharks opened the scoring through Talakai after Broncos winger Jesse Arthars spilt a bomb.
Kennedy scored his second with a classic after Hynes sent second-rower Briton Nikora on his merry way.
Trindall did the same for Hynes to stroll through for a 22-6 lead after Broncos hooker Cory Paix darted over from dummy-half for the softest of tries.
Mam scored on the cusp of halftime to keep the home side in it.
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