Starting underneath Adelaide Street at the Brisbane Square library and eventually passing just metres from blissfully unaware drinkers at the Criterion pub, the next stage of tunnelling works for Brisbane Metro is underway.

Brisbane City Council transport committee chairman Ryan Murphy said the 213-metre Adelaide Street tunnel would provide "a missing link in Brisbane's public transport network" connecting the South East Busway and the Inner Northern Busway for Brisbane Metro vehicles and buses.

"The tunnel will reduce the number of buses clogging CBD streets by up to a third, during the morning and evening peaks," he said.

"The Adelaide Street Tunnel will run underneath one of Brisbane's oldest streets, in amongst a complex network of utilities, services and heritage buildings, including Brisbane City Hall.

"In some sections, the tunnel roof will be just three metres below the surface, with the deepest point seven metres underneath Adelaide Street.

"To protect the surrounding streetscape, we have employed a delicate and precise construction methodology that will see the tunnel excavated in three sections."

Sixteen Olympic swimming pools worth of soil and rock will be removed to deliver the 213-metre tunnel, connecting North Quay with the King George Square Bus Station.

A purpose-built drill rig is being used during construction to install more than 20 kilometres of steel canopy tubes, which will help form the tunnel's arched roof section.

"The Brisbane Metro project is using a highly advanced tunnelling method, compressing the time we'll spend digging, from two and a half years to about 12 months," Cr Murphy said.

"This is the first time in Australia that this tunnel construction methodology has been employed in an urban environment.

"This tunnelling solution was chosen to minimise disruption to businesses, pedestrians, and commuters along Adelaide Street.