A second entry to the future Perth underground busport will be a tunnel under Wellington Street projected to take 60 buses an hour off the major east-west artery between Milligan and William Streets.

But it means more pain for tortured commuters along four-lane Wellington Street, which has been subject to roadworks for years, with one lane each way to be closed during construction.

Transport Minister Dean Nalder revealed details of the $20 million tunnel this morning, describing it as an expansion of the original scope of the project, to be funded from within its original budget.

“It wasn’t part of the original scope but what we found was that we had additional savings in building the busport and it allowed us to consider a more efficient way of moving the buses throughout this area,” he said.

The tunnel is to remove 40 per cent of the buses along Wellington Street by taking them all off the road between Milligan and William Streets.

It is expected to be operational by mid 2016 as part of the wider Perth City Link project.

“It will remove up to 60 buses an hour or a bus every minute,” Mr Nalder said.

“It will be a welcome move with the city commuters and motorists within the Perth CBD as it will remove so many buses off the streets.”

The two-way, 60 metre long entrance tunnel will begin about 50 metres west of William Street and westbound buses on the slope will be completely underground by the time they are parallel with Queen Street.

Buses heading east on Wellington Street can exit the busport the same way.

The existing four lanes of Wellington Street will be maintained once the tunnel is completed.

Mr Nalder said the original plan was for westbound buses to enter the underground busport from Wellington Street by turning right at Milligan Street near Perth Arena and accessing another tunnel.

That would have created a stream of buses waiting to turn right across traffic.

Shadow transport minister Ken Travers said he understood a tunnel to be envisaged since the project’s inception.

“I’m concerned that (former transport minister) Troy Buswell took it out of the project and Mr Nalder is having to fix up the mess that his predecessor left for him,” he said.

Mr Travers said it was a shame the Barnett Government had not been able to co-ordinate disruption to works along Wellington Street, particularly given Riverside Drive was affected by construction of Elizabeth Quay.

“I think people in Perth are getting pretty sick of disruption to Wellington Street because it’s now the main east-west link through the city.”