Health Minister Michael Ferguson said it had been a wonderful journey, with the development and expansion was progressing "extremely well".

"Our vision started in response to the need to do something better for families in the whole of the North, not just in the Launceston area," he said.

"Tasmania has never had dedicated adolescent facilities, in particular thinking of mental health needs."

The adolescent unit will have six single bed rooms designed to meet contemporary mental health care requirements. It will also have lounge, dining, and outdoor areas for patients.

Mr Ferguson said there would be a "consolidated stack" of services in one area, making more room for staff and giving them better opportunity to work as teams for women and children.

The precinct will be built in stages, with the first stage expected to be complete by October.

"Which will mean the adolescent unit will be available including mental health admissions," he said.

"This is a very, very exciting development and it's well overdue.

"The more we looked at this, the more we saw the opportunity for not just an extension of 4K and more beds in the adolescent unit, but the opportunity to build a women's and children's precinct," Mr Ferguson said.

"The government’s significant capital investment means we have been able to expand the scope of works included in the project. Construction works to complete the broader precinct are expected to be completed in 2020.

"The relocation of the Women’s and Children’s antenatal and postnatal outpatient clinics, which currently operate in ward 3D, will free up space in the hospital for more inpatient beds."

The cost of both the 4K redevelopment and women's and children's precinct will cost $20 million.

Mr Ferguson said parking was a concern.

"That's one of the reasons actually that led to this expanded development, which includes 38 spaces," he said. "We're also going to put another stack on the multi-story as well.

"It's amazing that we've been able to put a new building, a new footprint, and more spaces."

Ward 4K remains fully functional during the construction.