Police had been called to the property about 5.30am, and found a distressed 49-year-old woman outside with large cuts on her arms.
Officers forced entry to the house and found the man standing over the second woman, who appeared to have been stabbed in the neck.
Kapiti Area Commander Inspector Paul Basham said he was confident the two officers saved the woman's life.
"The two officers needed to take decisive action and one of the officers present deployed a Taser and the second officer deployed a Glock pistol, firing a single shot and wounding a 47-year-old male in the shoulder.''
All three people were taken to Wellington Hospital, where the man underwent surgery.
The two officers involved would be receiving welfare and psychological support, Mr Basham said.
"The application of lethal force is at the highest end of the continuum for us and it's a very difficult and challenging situation the officers found themselves in.''
Mr Basham said the officers acted with a "great deal of professionalism and a big dollop of bravery.''
The Taser used on the man was the old-style single-pronged Taser, not a new double-pronged version being trialled by police.
The incident will be investigated by an officer from outside the Kapiti Mana region and the Independent Police Conduct Authority.