IPCA chairman Sir David Carruthers said the officers should have left the bathroom and waited for Mr Reuben and his partner to get dressed.
"Instead they stayed in the bathroom and as the situation escalated the officers presented a Taser and [pepper] spray."
Mr Reuben and his daughter then went into the lounge and as he gestured to police to leave the house, he made contact with the arm of one of the officers, the report stated.
He was then Tasered by one officer while the other officer simultaneously pepper-sprayed him.
The use of the pepper spray was justified but Tasering Reuben was a disproportionate response and amounted to an "excessive use of force," Sir David said.
The report stated that one of the officers involved later claimed Reuben ripped off the shower door and threw it across the bathroom at the officers.
The same officer also claimed Reuben used his daughter as a human shield to prevent from being Tasered.
Both claims were contradicted by the Taser's camera footage and later resulted in Judge Joanna Maze dismissing a charge of assaulting a police officer at the Timaru District Court.
Assistant Police Commissioner Allan Boreham today apologised to Mr Reuben and his family, "particularly for the distress caused to his children".
The serious issues the report raised were being investigated and respective employment and criminal investigations into the conduct of all staff involved were under way, he said.
The officer who presented the Taser at Reuben later resigned from police and the other officer involved would remain on full, active duties while the inquiry took place, Mr Boreham said.
Police launched an internal inquiry following the initial incident, which would be reviewed in the wake of the IPCA report, he said.
"At that stage they didn't find that the police had done anything wrong."
Neither of the two officers highlighted in the IPCA report were stood down following the incident, Mr Boreham said.
Depending on the findings of the independent inquiry, repercussions ranged from no further action, to further training and internal penalties - including dismissal - to legal action, he said.