Inspector Simon Feltham said it was not known how long the woman had been in the water, but given her situation she would have been unlikely to survive if officers hadn't responded and found her so quickly.
"This was a difficult situation with a large area needing to be covered in a short period of time," he said.
"I'm very proud of the response from police staff who came from across Marlborough to this search and am rapt that they were able to successfully locate the elderly woman in such a short period of time"
The hospital today reported the woman was in a satisfactory condition.
Ashwood Park Retirement Village manager Ross Bisset said the rest home followed all District Health Board instructions on the woman's care.
"She's free to come and go as she pleases."
Mr Bisset said the woman was found about 100m from the rest home.
"The stream had about one foot of water in it," he said.
"By all accounts she's recovering in hospital."