Then all of a sudden, Briggs said, "something happened where they all kind of moved in one motion and banged into the door and the latch just popped open".
"I remember that really clearly because I'd been watching so closely," he recalls, 33 years later. "Out sprang these lions and my brother and I were just ... well, we shat bricks basically. It was unreal, it was shocking. It was just like, 'Holy s***'."
Briggs and the other spectators stayed put "for what seemed like forever" before eventually being released by staff. They then sprinted the whole way home without looking back.
"I kept waiting for lions to jump out from behind every car. It was a terrifying thing, but one of my greatest memories," Briggs said
But that's not even the whole story. Briggs heard on the radio the next day that one of the circus' performing poodles had been locked in the lions' cage before the show, and the kill had frenzied the lions to the point that they were worked up enough to escape.
"As I've grown up and gotten older, this footnote of the poodle has become more and more fascinating because in my mind there was this real human drama which would've led to someone putting a live poodle into this lion cage," Briggs said.
That's why, as an adult, he's now made a seven-part, Making-A-Murderer-esque documentary series, uncovering the great mystery of Who Killed Lucy the Circus Poodle? In it he follows the trail from his own memories to an old article, to the journalist who wrote it, and the information she sent him - "and the investigation took off from there".
"It is a true crime doco about a murder cold case ... it's just that the victim happens to be a circus poodle.
"It was everything I hoped it was going to be and more. It was fascinating; every new turn, every new person, every new bit of information was really compelling. It's a real rabbit hole, man. It was such a great journey."
There are twists and turns too as Briggs factors in everyone from the "kingpin of New Zealand's circus world, Mr Tony Ratcliffe" to the then Prime Minister, David Lange who, it turns out, is "a big turning point in the investigation".
"I can't say why and spoil it but ... he's just intrinsically linked to the fate of the investigation, he has a big part to play," Briggs teased.
The entire project was a first for Briggs, who has worked in television for years, however this was his first time leading an investigation and fronting the actual series.
He describes it as "an enormous undertaking", but a truly rewarding experience because "I had a great yarn, but it was a story without an ending, and now it has one."
"I feel great. I've got goosebumps just talking about it. It's a mixture of my childhood, this astounding event, my brother, my family, the time and place I grew up in and this tale that over the years has become like a faint memory or dream. And being able to go back through it and unearth the final part of the story has just been a massively rewarding journey."
Who Killed Lucy the Poodle will be available to stream in full on TVNZ On Demand from May 21.