Northland FC equalised 10 minutes later as Schimmel headed home a free-kick.
Josh Nishitani-Hart gave Northland the lead three minutes later after cutting inside of his marker and nailing the shot from inside the box.
Jordan Higgins equalised for Lynn Avon right on halftime.
The visitors were closing in on promotion but Schimmel decided enough was enough and delivered a second-half hattrick.
He netted his first one shortly after the restart before adding two in four minutes to clinch the victory.
Nishitani-Hart added a second for himself in the 83rd minutes to extend Northland's aggregate lead in the two-match tie and secure Northland FC's position in NRFL Division Two.
Johnston said he was glad his players got up for the win to keep them in the highly competitive Division Two.
"I was extremely proud of the players and was in no doubt that they could come back from the 4-3 defeat in Auckland last week," he said.
"The key to our victory on Saturday was the way we quickly transitioned from defence to attack. Lynn-Avon struggled to deal with the pace and quality of our forward passing and movement.
"This result will give us some confidence leading into our first league game on Saturday against Te Atatu at Tikipunga but we are fully aware of how ruthless Division Two can be if unprepared.
"We will be working hard in training to ensure we are well organised and structured so we can hopefully make a good account for ourselves this season."
Northland will be hoping for a better performance in the division after last season yielded just two wins from their 20 games and 61 goals conceded.
