The DHB's chief executive, Margie Apa, is appalled by the assaults, which happened at 7.05am.
"The attackers were hiding behind a wall and I understand knocked one staff member down from behind, and broke the collarbone of the other staff member. The attackers then ran from the scene."
"These were two members of staff who had come off night shift. ICU, the department where they work, is an important service that provides the most intensive care for patients who are the most vulnerable."
"We are appalled and devastated that any of our staff, all of whom dedicate their professional careers to saving lives, have to experience this kind of unprovoked violence."
The DHB said the attackers' identity was obscured from cameras by fog. Security officers are trying to find other security camera footage.
"Middlemore has had increased security at its carpark sites since May last year, with a further increase in security staff in December."
Apa said: "We are asking our community and in particular, those who live in and around the Middlemore train station area, to come forward if you have any information that can help us identify the offenders.
"In the meantime, Counties Manukau Health has again increased our security presence at the early morning shift changes, in addition to the afternoon and night shifts."
A police spokesman said the incident appeared to have happened on Gray Ave.
"... the offender(s) reportedly left the area immediately after the incident. At this stage Police have not received a complaint about the incident."
A spokeswoman for the NZ Nurses Organisation, Hilary Graham-Smith, said, "It is intolerable that health staff are assaulted."
"I think most hospitals have beefed up their security presence over the last 12 months because of hospital staff being attacked - outside hospitals, in emergency departments, on wards, wherever," said Graham-Smith, an associate professional services manager for the union.
• The DHB urges anyone with information about the attack to call the police or Middlemore Security on 021 242 5473.