Last year, while still in year 12, the Otago Boys' High School pupil finished NCEA level 3 calculus and gained a New Zealand Scholarship award for calculus - something usually achieved only by New Zealand's top year 13 pupils.
And this year, the 17-year-old is studying maths at the University of Canterbury by correspondence.
Johnny said he had competed in the Eton maths competition twice before, but this was the first time he had reached the national final.
He is one of 15 finalists out of about 1000 entries from across New Zealand.
''I was completely shocked when I first found out I'd made it.
''The test was a lot harder this year.''
The preliminary paper was essentially a problem-solving exam which tested algebra, trigonometry, geometry and probability, he said.
The final, a two-hour examination, will be held at Hutt International Boys' School on August 21.
''The finals exam will be the same thing, only harder,'' he said.
Maths competitions are not new to Johnny.
During the past five years, he has achieved high distinctions in the International Competitions and Assessments for Schools (ICAS) maths competition, he has been in the top percentile of the Australian Maths Competition, and has been a member of the 2013 and 2015 New Zealand Mathematics Olympiad residential training camp.
But the Eton competition was new territory for him, so there was no pressure, Johnny said.
''I don't have any expectations to live up to, so I'm hoping it will go great.''
