"It has been fantastic to look around and see no fences and no barred windows, but instead find large, comfortable communal spaces, open courtyards, and a whare out front," he said.
Originally budgeted at $53.6m after a tender process, the first stage was originally due to be completed in time for patients to move in last December.
But that was later pushed back to September this year due to "challenges in the Auckland construction market", according to Counties Manukau DHB.
The DHB also confirmed to the Herald in August that the cost had risen to $56.9m.
Clark said the new facility increased badly needed capacity for acute inpatient mental health treatment in Counties Manukau.
"Across New Zealand, there is still much progress to be made in mental health care. We know that there are significant challenges and people whose needs are not being met. We also know that these challenges disproportionately affect particular communities, and that many of these disparities are unjust and preventable," he said at the opening today.
The minister's press release said stage two was expected to begin in early 2019, but the DHB told the Herald in August it was scheduled for completion by April next year.
Ebert Construction, which went into receivership in August, had been the main contractor for the development of the new Tiaho Mai mental health facilities.
It also emerged in June that the existing Tiaho Mai facility had problems with leaks.
Documents released by the health board under the Official Information Act showed former health minister Tony Ryall was told in 2012 Middlemore had more than one leaky building, including the mental health unit.
Funding for the rebuild was allocated in the 2014/15 financial year.
