He joined the Defence Force in 2007 and was embedded in a patrol team as a medic in Afghanistan in 2010.
For today's ceremony, Mr Sweetman wore his army medals, including his three-year Service Medal.
His time in the Army had helped him in some aspects of his police training over the past 18 weeks, he said.
"I think I had a bit of an advantage in the shooting and maybe in the marching as well," he said.
He aspired to reach the rank of detective and perhaps join the Armed Offenders Squad, he said.
The officer continued a family tradition of working in emergency and armed services.
His twin brother, who was in the army, was now a fireman based in Auckland and his father, also in Auckland, was a police prosecutor.
"My grandfather did compulsory military training and my great-grandfather was in World War I.
"So we've all done some military service."
The squad's patron was Invercargill Mayor Tim Shadbolt, who was dressed in his full mayoral robes to acknowledge the special occasion.
He noted the importance of police in the community.
"We simply could not provide the safety that our communities expect without you."
Another graduate, Leadership Award winner and second in wing Constable Ann Tapara credited five years of studying psychology for developing a sense of empathy, which she believed would be valuable in her police career.
"It's helped me to understand the reasons behind why people do what they do and still respect people despite their behaviour," she said.
Ms Tapara hoped to get in to child protection, as she said it was what she was really passionate about.
"It fires me up when we talk about it and makes me want to do something."
Police Commissioner Mike Bush told the graduates their chosen career would give them a "real sense of purpose".
Police Minister Anne Tolley told the group they had joined a police force that was respected worldwide.
"Our police are out spending more time in the community preventing crimes," she said.
