Sergeant Anthony Harmer said there had been many possible sightings of Berrington and those were being verified.
They knew Berrington had attended a church dinner on Tuesday night but was known to be at home at 9pm.
Today the Police Dive Squad searched the Hutt River from the Totara Park overbridge south for any sign of the missing woman or for indications that she may be in the river.
Sergeant Harmer said the search effort, which started on Wednesday, may soon be scaled back.
"We have spoken with Mary's family this afternoon and while the search will continue tomorrow we are coming to a point where we may need to scale back efforts as searches have been completed of all reasonably possible areas where Mary could be," he said.
"Police and Mary's family would like to thank everyone who has supported the search operation so far."
Berrington has mental impairment and may not be in a position to help herself, police said. She may also have stopped taking her medication.
Berrington went missing in 2008 but was found days later.
Police said she was believed to be wearing a dark polar fleece jacket and dark leggings.
• Anyone who has seen Mary is asked to call police on (04) 527 2300 or call 111 immediately.