Johnstone has ridden four winners since Sunday, including a double for Maher at Sandown on Wednesday, and has a strong book of eight rides at Flemington on Saturday, of which seven are for Maher.
One of his main rides comes early in the day on Kaleo in the Play On, For Neale (1600m).
Jack Turnbull, the National Assistant Trainer for Ciaron Maher Racing, said one of Johnstone's biggest assets was being a natural lightweight and being able to fully utilise his 3kg claim while also being a strong left-handed whip rider.
"When you can take three kilos off, and you're of his stature as a rider, being left-handed, it's an asset," Turnbull said.
"All those little factors add up and I'm sure he will be an asset to the stable over winter."
Turnbull said the stable had little hesitation in keeping Johnstone aboard Kaleo on Saturday after the apprentice rode the gelding when second at Sandown behind stablemate Siriusly Hot over 1600m on May 23.
Kaleo has finished second in both outings this campaign after being tried in better company while a colt during the spring.
His solitary win came at Caulfield in May last year after which he was transported to Brisbane for a shot at the Group 2 BRC Sires' Produce Stakes (1400m).