He intends to expand the battalion from 147 to 250 members in the next three years by raising the unit's profile and drawing people from the pool of "smart, academically qualified, industrious and hard-working" young people Otago and Southland have "in spades".
Luckily for the battalion, he is not all talk.
After a 25-year career in the army, with stints as a military observer in Bougainville, three years based in Bangkok as the New Zealand Defence Force's attache to Thailand, Cambodia, Laos and Vietnam, an eight-month tour in 2007 as the contingent commander of New Zealand forces in Afghanistan, and two years in command of the Waiouru army camp, Lt-col McElwain is a man of action.
However, he acknowledges that, at 50, he is nearing the end of his tour of duty and is happy to step aside to let younger soldiers come through into leadership positions.
He said he planned to leave the army in about three years and spending those years in Dunedin - after eight years as a territorial force soldier with the 4th Otago Southland Battalion Group between 1977 and 1985 - was an ideal note to go out on.
"It's with great pleasure that I have come back."
Born in Mosgiel, Lt-col McElwain spent his school years in Gore, before completing a science degree at the University of Otago.
He joined the regular force as a lieutenant in 1985.
Returning to Dunedin as commanding officer of the battalion meant the rare move of accepting a drop in rank - from full colonel to lieutenant-colonel - but that did not put him off.
"The other option was something in Wellington and I was not interested.
"I wanted to give something back to the unit that I still think, 25 years later, gave me my grounding for my military career.
"I started here and I would like to finish here."
He also had family here and Dunedin was a great place for him and his wife, Anne, to continue raising their three children, the youngest of whom is aged 12.
He was also keen to further his interest and involvement in Forest and Bird and ornithology.
Lt-col McElwain replaces Lt-col Grant McMillan, commander of the 2nd Canterbury Battalion Group, who has been 4th Otago Southland's interim commanding officer since June last year when then-commanding officer Lt-col Grant Day left after three years to take up a position on the general staff at defence headquarters.
An official handover to Lt-col McElwain will take place during celebrations for the 150th anniversary of military service in Otago and Southland later this month.
