It marked the third time a semi-pro team has beaten an A-League side in the FFA Cup - Adelaide City toppled Western Sydney Wanderers two years ago, while Green Gully defeated Central Coast Mariners on Tuesday night.
Redlands boss Matt Chandler believes it's proof there is plenty of untapped talent below the top tier, plugging away in the National Premier Leagues (NPL) - if A-League coaches care to look.
"It's amazing, now, for Australian football that the NPL is now producing more and more teams that can compete with A-League teams in a one-off battle," Chandler said.
"I do feel there's a number of players in the NPL that could actually play in the A-League.
"Especially in Queensland we're underestimated - obviously in Melbourne and NSW there's some superb sides, but there's a number of good sides that play in this league.
"We're sitting fourth or fifth, that shows the depth of the league in this state."
Chandler could barely wipe the smile off his face after the historic victory, which seemed out of the realms of all possibility when Ryan Kitto put Adelaide ahead on the hour mark.
Redlands had spent most of the night on the back foot, camped in defence and struggling to keep Guillermo Amor's men at bay.
But a howler from Eugene Galekovic opened the door for Queensland's second-oldest club, and Michael Lee swept into an open net to level the scores in the dying moments.
Belief grew from there.
Lee then provided the cross for Paul O'Brian, who put Redlands ahead eight minutes into extra time, and they hung on for dear life to the final whistle.
"It's a huge achievement, we can't underestimate what we've just done here tonight," Chandler said.
Redlands will now advance to the round of 16, and Chandler believes few teams will want to face them now.
Amor, meanwhile, said Adelaide was disappointed, but not embarrassed.
"This is football. Sometimes, it can happen - it shouldn't happen, but sometimes it does," he said.