The 79-year-old victim was found dead in her Khandallah home on January 24, and police earlier said she died due to a "violent act".
In a statement this evening, Detective Senior Sergeant Tim Leitch said the woman was taken into custody and charged this morning.
Det Snr Sgt Leitch said it was a "significant development in the investigation and our thoughts are with Helen's family at this time".
The charged woman will appear in court tomorrow.
Police searched the Porirua landfill on Friday and Saturday for items they believed were discarded several hours after Gregory's death.
They are now searching the home of the arrested woman.
"Police would like to acknowledge the response and support of the community during the investigation," Det Snr Sgt Leitch said.