The van had since been removed from the building which had no structural damage, she said.
Work and Income Waikato Regional Commissioner Te Rehia Papesch said police were called after a threat was allegedly made against staff.
"Our team responded promptly, moved people in the building to safety and temporarily went into lockdown while police worked to resolve the matter."
There had been some damage to the building which was being assessed to determine when the building can reopen.
"When security incidents occur at our MSD offices, we have a standard process we follow to protect staff, clients, our security guards and members of the public.
"We're sorry for any inconvenience this may cause our clients today – we want to encourage anyone needing help to give us a call on 0800 559 009 or use their MyMSD app."