Mr O'Reilly said the man had no obvious injuries and he saw nothing which made him think there had been a fight, such as blood on the ground.
A blue tent is being erected in Victoria Park by police to house evidence in case of rain.
Auckland City Police Detective Sergeant Kevin Blackman said all items of interest would be put inside the tent to avoid water contamination if it rained.
He said there was no obvious weapon found at the scene, "but then what's a weapon. We don't want to rule anything out."
He said the man's injuries were so severe they could be life threatening.
The man was still unconscious in hospital.
Ambulance crews arrived at the scene earlier, classifying the man, believed to be in his 30s, as status one -- critical -- and administering initial treatment before transporting the man to hospital.
Mark Bowater, Auckland Council parks manager, said: "Our parks maintenance contractor for Victoria Park found the man unresponsive and immediately dialled 111."
They stayed to support the victim until police and emergency services arrived just before 8am. We will assist the police however we can with their investigation.
"The staffer who found the victim is doing okay and will be supported through the council's employment assistance programme as required."
A witness said she could see a police cordon within the park near Fanshawe St bus stop.
An Auckland DHB spokeswoman said the man was still being assessed and an updated condition would be available later today.