Brisbane's CBD was hit by transport chaos on Wednesday morning, with emergency roadworks and several accidents causing major delays across the city.
Brisbane City Councillor Ryan Murphy said there was a "void" found below Adelaide Street around 4am while contractors were excavating a tunnel as part of the Brisbane Metro project.
"Contractors were doing piling on the Adelaide Street Tunnel when they noticed some subsidence behind the tunnel piles underneath Adelaide Street," he said.
"A decision was made out of an abundance of caution to close Adelaide Street between North Quay and George Street.
Mr Murphy said the area was one of the oldest in Brisbane.
"We have to be conscious that this is one of Brisbane's oldest streets and we're always going to find weird and wonderful things under there," he said.
"Unfortunately, last night we discovered a void and that's had a really big impact on Brisbane's road network this morning, which we apologise for.
"It may have been there for many years but obviously once it's been discovered it is our duty to make sure that it is safe for commuters and that's what we'll be doing this afternoon – backfilling it with concrete."
Earlier today, Translink informed commuters that all CBD buses were delayed due to congestion, with some passengers told their buses would divert to nearby train stations due to a "massive backlog into the city".
But by 3:30pm on Wednesday, Adelaide Street had been reopened and all bus services have returned to normal.
Some commuters were stuck in buses for more than an hour while others were turned away from boarding buses in some locations.
Police motorbikes were dispatched to Adelaide Street and George Street in an attempt to get traffic flowing.
There were also complaints about drivers blocking intersections, exacerbating the issue.
Tunnel may be 'over-excavated': Union
Rail Tram and Bus Union assistant state secretary Tom Brown said the area under Adelaide Street may have been over-excavated.
"A very reliable source informs me that they've actually over-excavated," he said.
"The story of the void there doesn't seem to stack up. Surely the engineers would have picked that up with the ultrasound."
Mr Murphy said it was "not expected" and "it would not happen again".
"We have actually used ground-penetrating radar to look beneath Adelaide Street ... obviously this has been missed somehow, or wasn't detected, these things can sometimes happen.
"The chances of this happening again are very, very low."
Mr Brown said traffic congestion throughout the morning had also resulted in bus drivers being verbally abused.
"Every driver basically in the network is running late … it's going to cause chaos for at least the rest of the day.
"The abuse levels for our drivers is already at unacceptable levels," he said.
"The drivers have been instructed to drop people off at the closest train stations … so people can basically make their own way into the city, and that is not going down well."
