He was a junior member of Brent Ward's sprint stable last year but is now confident of matching the senior members of the squad.
Johnston (17) dominated the men's sprints at the Caledonian Ground on Saturday.
There was daylight between him and the next runner when he won the 100m in 11.14sec, and the 200m in 22.91sec.
The strength he displayed pointed to lessons learned during the Commonwealth Youth Games in India and the Pacific Schools Games in Canberra.
Having won four individual sprint silver medals at New Zealand championships, Johnston will be seeking the elusive gold medal in the men's 19-and-under grade at the nationals in Christchurch at the end of March.
It has been a good week for the family because his father, Ken Johnston, was elected a life member of the Otago Children's Committee.
Ken Johnston officiates at the Caledonian Ground each week for children's and senior athletics.
The other sprint highlight came from 31-year-old Anna Smythe (Hill City), who scorched away from her younger opponents to win the women's 100m in 11.87sec.
She won silver medals in the 100m and 200m at last year's national championships.
The fields on Saturday were boosted by 30 athletes who had travelled from Invercargill to test themselves against their Otago opponents.
One of the most impressive performances came from Matt Robinson (STPL), who won the 400m in 48.01sec to beat Toby Flett (Hill City), 49.61sec and Glen Ballam (Southland), 50.28sec.
Robinson and Ballam are both coached by Lance Smith in Invercargill and are training for the New Zealand secondary schools championships in Timaru next month.
Robinson (18) has won the senior boys 400m title for the past two years and is keen to complete the hat trick of titles.
Ballam's best chance is in the senior boys 800m.
He demonstrated his ability when he came from behind to dead-heat with Callan Moody (Ariki) in the men's 1500m in a Southland age-group record of 3min 58.51sec.
Moody took off down the back straight and looked to have the race won when he led by 3m before the final bend.
But Ballam sprinted strongly into the straight and caught Moody 60m out.
It was a tight battle to the tape, neither runner yielding as they crossed the line together.
The last lap was completed in 60 seconds, and Ballam and Moody were both timed at 3min 58.51sec.
Four Otago records were broken on Saturday.
Rebekah Greene (Hill City) lowered her own Otago record for a 15-, 16- and 17-year-old by seven seconds when she won the women's 1500m in 4min 29.76sec.
Anna-Lisa Uttley (Ariki) set an Otago record for a 16-, 17-, 18- and 19-year-old when she won the women's 5000m in 17min 16.13sec.
Jim Flynn (Leith) broke the masters men's aged 75 to 79 record standard by 40 seconds when he ran the 1500m in 7min 20.35sec.
Mackenzie Haugh (Hill City) broke his own Otago aged 13 high jump record with a leap of 1.78m.
