McKellar made a plethora of changes for Saturday's encounter at HFC Bank Stadium in Suva, but was dealt a late blow when the Wallabies' utility back Joseph-Aukuso Suaalii was ruled out with hamstring tightness weeks after returning from a two-month-long hamstring injury.
He reaped the rewards of his team's transformation though, as the Waratahs scored eight tries to five to end a dismal 15-game losing streak when playing outside of Australia.
The bonus-point win, which is also their first victory in Fiji, keeps the Sydney-based team's finals hopes afloat, having lost seven of their past nine encounters going into the match.
They now sit two points adrift of the sixth-placed Queensland Reds with two rounds remaining.
"That (win) means we stay alive, we give ourselves a chance, and it builds the confidence and belief in the group with what we're working on," McKellar said.
McKellar missed out on finals last campaign after taking over NSW ahead of the 2025 season, and a loss on Saturday would have dealt a huge blow to their post-season hopes again.
Instead, the victory sets up a massive clash against the Brumbies next Friday, where the Tahs will look to thrust themselves firmly into the finals fray.
"The whole change in the process started about 18 to 20 months ago, and we knew there'd be some tears along the way, and there have been," McKellar said.
"It's great for the players and the staff to be able to enjoy what was a good performance, but we crack on."
Fresh off a bye, the Drua needed a win to keep their finals hopes intact, but leaky defence, along with a sea of turnovers, cruelled their chances.
"We wanted to start fast, but unfortunately it took us the whole 40 minutes of the first half to actually get our rhythm," co-captain Temo Mayanavanua told FBC Sports.
"There were a couple of key moments that we weren't urgent enough in."
The Waratahs crossed first when Harry Potter got on the receiving end of a sneaky pass from Ioane Moananu, before Moananu scored his own five-pointer when he capitalised on a rolling maul.
Max Jorgensen pounced on a Jack Bowen grubber in the 19th minute, but the Drua responded through Mesake Doge.
Teddy Wilson, Sid Harvey and Angus Scott-Young piled further misery on the home team, each scoring as the Waratahs opened up a 36-7 lead at the break.
Moananu and Fijian Apolosi Ranawai wrestled over for the Tahs in the second half, but the Drua's Elia Canakaivata, Kitione Salawa, Tuidraki Samusamuvodre and Mayanavanu salvaged some respectability for the hosts.
