Cundy produced a personal best as the men's 4x100m team smashed their own world record on the way to glory.
Kenny Cairns gave Britain another gold in the pool when he retained the men's S3 100m freestyle title in a new Paralympic record.
There were also wins for Lee Pearson in the individual dressage and javelin thrower Ken Churchill in the F37 classification.
What made Cundy's swim even more special is that he is a butterfly specialist.
After soaring past the Canadian swimmer to take the lead, he brought the team of Matthew Walker, Marc Woods and David Roberts home in 4mins 6.85secs, with a personal best time of 58.9secs.
Afterwards he said: "I knew the Aussie was three seconds faster than my personal best. When I saw he hadn't passed me by the turn I knew I could do it."
Cairns was another whose self-belief propelled him to victory.
The Scot, who clocked 1min 54.14secs to break the record, admitted: "I felt it would be close at the turn and I didn't know what kind of lead I had but I knew I had turned quickly and just went for it."
Staffordshire-based Pearson, who is the current dressage world champion, rode Chipchaseknes to a score of 67.96 points to finish a point ahead of Denmark's Brita Anderson.
Britain's other rider, Dianne Tubbs, on Jaenaadi Jedi, finished in sixth position with 65.18 points.
Pearson said: "I'm gobsmacked but ecstatic. I didn't think today would be my day. I wasn't feeling too good yesterday or this morning, but this has lifted me."
Remarkable
The feat was made the more remarkable considering Pearson only found out he was riding the horse six days ago.
"He is quite lethargic, not what we in the trade call a push-button mount," he said. "I had to do a lot to keep him up to his work."
There was disappointment, though, for Thomas Evans, who finished just five seconds outside the medals in the 5km cycling time trial.
Evans, who missed out on the medals in Atlanta, finished just behind bronze medallist Marius Louw of South Africa in a time of eight minutes and 26 seconds to end his medal hopes in Sydney.
He said: "I rode it as hard as I could and all the lines were perfect.
"I'm just so disappointed as it was my dream to win gold. I was giving it maximum effort all the way.
The gold eventually was won by Yong-Sik Jin of Korea in a time of eight minutes and 15 seconds.
"It is the one title I don't possess so it was my dream. I'm gutted," the Corby rider said.
Britain's dreams of gold in the men's wheelchair basketball event were ended by the strong Canadian team, who maintained their unbeaten record to advance to the final.
Britain crashed 66-51 and now face a bronze medal play-off with the losers of the USA-Netherlands match.