Hospital staff could be seen inside the hospital, and patients took to social media to speak of their experience at the time.
The lockdown ended ay 6.15pm, following "extensive inquiries" at the hospital and in the surrounding areas that determined there was no threat to hospital staff or the public.
"The hospital was locked down until police were satisfied there was no real threat."
Hawke's Bay District Health Board spokesman Dr Colin Hutchison said the hospital activated its internal emergency response procedures which worked "well" resulting in a swift response from police.
"The number one priority at all times was to keep staff, patients and visitors safe."
A triage unit for people who needed emergency care was set up in "record time" in another part of the hospital as police responded to the threat, he said.
"The DHB would like to thank St John for rapidly responding to the situation enabling the public to continue to receive care offsite from the main Emergency Department at other acute and medical centres."
Hutchison also offered his thanks to police for their "timely response and leadership during this event".
"As with any emergency response event, support has been offered to staff and debriefs are taking place."
Police would not elaborate further on the nature of the threat or whether it was associated with the Christchurch mosque shootings.
To date, the investigation has not identified the author of the note and further inquiries were on going, the spokesman said.
