McKellar was asked by Wallabies coach Dave Rennie last year but declined, preferring to focus on the Brumbies.
But after taking his team to their second successive Super Rugby AU final – for a win and a loss – he has opted to sign on for the international season.
He will return to Canberra before the 2022 Super Rugby campaign but is being earmarked for a full-time role with the Wallabies heading into the 2023 Rugby World Cup.
Australia return to international action in a three-test campaign against France next month, with game one on July 7 at the SCG.
