Whanganui District Health Board chief executive Russell Simpson said the hospital had enacted full lockdown procedures from about 2.30pm and staff had talked to police to assess the level of risk.
At just after 5pm, Simpson said all services had resumed and the campus had been cleared.
"We did have a few disgruntled people who couldn't get in or out, and whilst I appreciate the inconvenience to them, their safety is the main concern for us, as is the safety of our own staff.
A Whanganui Chronicle photographer on the scene said there were no police cars visible in the area at the time of the lockdown, but it was clear people couldn't get into the hospital building because the doors were not opening.