A new state-of-the-art mental health facility is being described as critical for the growing population on Auckland's North Shore.

The $25-million, 46-bed development has been officially opened today.

Until now, mental health services have been delivered out of a decrepit 1950s ex-maternity unit building at North Shore Hospital.

Waitemata District Health Board chair Dr Lester Levy says mental health is a major issue, and the new unit will meet the needs of the community well into the future.

"Between the facility that we have here and the facility that we have in West Auckland, we've probably future proofed for the next two decades."