Board chief executive Kevin Snee said the current mental health unit, built in 1978, was "not really fit for purpose" and its replacement was "long overdue".
Work would begin immediately on developing a new service model and the facility was expected to open in the 2014/15 financial year.
"It's a $20 million project, it's not going to be built tomorrow," Dr Snee said. "It's going to take some time tendering and commissioning and so on."
Disability Support Advisory Committee chairwoman Diana Kirton said the board could "revisit and refresh" earlier plans from when the project was last considered in 2008.
She said those site-development plans included integrating the mental health unit with the main hospital building to give greater flexibility of services and better access to other departments.
The committee would discuss the new unit at next month's meeting and later make a submission to the board about how to build a space "where patients, staff and visitors feel safe and comfortable, unlike the atmosphere created in the totally outdated facility we currently have".
Mrs Kirton also emphasised the importance of consulting with patients and their families during the development process.
A family member of one of the unit's patients was "very pleased" with the news.
The man, who wished to remain anonymous, had been lobbying the board for a new mental health unit for the past five years. He was hopeful the unit would be mainstreamed with the hospital's other areas so patients "wouldn't feel like outsiders".
Hawke's Bay National MPs Chris Tremain and Craig Foss said the hospital sale was also a win for Napier. "The derelict old buildings have been sitting there accumulating huge holding costs," Mr Tremain said. "This provides a wonderful opportunity for Napier city to move on and for a modern development to take place."
Todd Property Group's plans for the site are unknown. Sifa Taumoepeau, manager, corporate and government affairs, said: "It's a great site for sure and we will look at all the options over the coming months."