Emergency services were called to the Norfolk estate on 8 July after reports that an estate worker had fallen into the Lower Lake while operating a ride-on lawn mower.
He was flown by an East Anglia Air Ambulance helicopter to Addenbrooke’s Hospital, Cambridge, with what were described as life-threatening injuries.
A spokesman for Norfolk Police said: "The man was retrieved from the water and was taken to Addenbrooke's Hospital by air ambulance for treatment. He is currently described as being in a critical condition.
"Police and the Health and Safety Executive are carrying out a joint investigation into the incident."