The winger's rollercoaster season hit its highest point last Friday night when he ran in five tries during Souths' extraordinary 60-8 rout of the Sydney Roosters.

It allowed Johnston to claim his second Ken Irvine Medal, adding to the top try-scoring gong he won as a rookie back in 2014.

The turnaround has been dramatic for the 25-year-old and the club, with the Rabbitohs looking at letting Johnston walk earlier in the season.

Melbourne and Wests Tigers all had cracks at luring Johnston away from Redfern until Souths backflipped and offered him a new two-year deal three weeks ago.

"Mentally, it was a bit of a rollercoaster ride for me. A lot of ups and downs," he said.

"The longer the season went, the more it was playing with my mind.

"But I was pretty stoked to have some interest from other clubs. To finally sign the contract was a huge relief.

"It was very close (to leaving Souths). It pretty much came down to a couple of days there, where it was pretty much 50-50.

"Pretty weird to think about it now. I gave myself every chance to try and stay in Sydney, and stay with Souths.

"Once it came through that there was a contract, I was pretty stoked and couldn't wait to put pen to paper."

But Johnston's focus is now on the Rabbitohs' finals campaign as they prepare to take on Newcastle at ANZ Stadium on Sunday.

Johnston is hoping Souths can carry the confidence of a 52-point demolition of their rival into another finals campaign.

"It was very pleasing to play the way we know we could," he said.

"Last game of the season, you want to go out with a bang and lead into finals footy playing like that definitely gives us a lot of confidence."

The Knights defeated the Rabbitohs in their only encounter this season, winning 20-18 back in July.

Australian Associated Press