The victim suffered extensive facial bruising, the hearing was told.
Lt Saua's commanding officer, Major Faraday, described him as an "enthusiastic and professional'' soldier, and believed the incident to be out of character.
Giving evidence this morning, Lt Saua said he had no history of violence before the incident - a yellow card during a rugby game was as bad as it got.
Lt Saua was deployed to Afghanistan between July 2011 and April 2012.
Asked whether he had seen action, he told the court his first patrol was attacked with a rocket-propelled grenade and described his involvement with the identification and arrest of a "person of interest'', in conjunction with local authorities.
The hearing was told that following a restorative justice session, and given Lt Saua's contrition over the incident, the victim was more understanding of the incident's circumstances.
Lt Saua is one of three Samoan officers in the New Zealand Army. His family lives in Palmerston North, although his father is a village chief in Samoa.
The court heard Lt Saua has a strong involvement in his native culture; he speaks fluent Samoan, and has previously caught media attention for his extensive pe'a - traditional Samoan tattoos.
Lt Saua expressed his intention to leave the Army, perhaps to study at university, but said he wished to remain a reservist.
Commanding officers at the court martial described Lt Saua as a professional, hardworking soldier and his assault on another officer was out of character.
Judge Gaskell said Lt Saua's early guilty plea and his previous clean record went a long way to reducing any sentence.