With 20 minutes remaining Sinclair added his second and Team Wellington's fifth, his curling cross-come-shot nestling perfectly in the top corner, before another replacement Hamish Watson nodded home the sixth with seven minutes left.
"When I came on I just wanted to make a difference, try to score a couple, work hard and press the opposition," said Sinclair.
"It was always going to come. We had a few good chances in the first half. We were much more clinical in the second half," he said.
On a cool, calm day in misty, yet persistent rain in the capital, Team Wellington completely dominated the game, mounting wave after wave of attack with Bevin, Eric Molloy and Nathaniel Hailemariam particularly prominent and Roy Kayara a constant threat down the right.
It was no surprise when the home side went ahead after 16 minutes, courtesy of Allen. The prolific Guernsey marksman has built a career on being in the right place at the right time and the opener was that of a master goal-poacher. Allen found himself on-hand to gleefully stab home from close range after Kayara's thunderous drive cannoned off the crossbar.
Only poor finishing prevented Team Wellington posting a far more comfortable half-time lead, but their second-half performance rendered that inconsequential. Lautoka FC's approach was ultra-defensive and typically physical and while that worked for the first 45 minutes, they were simply outclassed after the break.
"We knew we had to keep calm and keep patient and eventually one of them has to go in," said captain Justin Gulley.
"We had so many chances and the law of averages tells you that one of them, two of them, maybe even five of them are going to go in. We just kept doing what we were doing and knew they would come."
Team Wellington now have the seemingly straightforward task of completing the job and earning a first ever appearance at the FIFA Club World Cup when the two sides play the second leg in Lautoka next Sunday. But any thoughts of complacency have been quickly put to one side.
"Next week the aim will be to try to do that again over there," said coach Jose Figueira.
"I think you've seen this season the boys just love playing, getting out there and expressing themselves. If we show the same application and effort they showed for each other today, we should be fine."
OFC Champions League final, first leg
Team Wellington 6 (Sinclair 2, Allen, Bevin, Barcia, Schrijvers)
Lautoka FC 0