At the world championships in Kazan, Russia this month, she picked up two silver medals, in the 800 and 1500m freestyle, both times being beaten only by the freakishly gifted American teenager Katie Ledecky.
That took to five her world championship medal haul, one more than double Olympic champion Danyon Loader, and a New Zealand record.
The last four years of her career have taken a steady upwards path.
At the London Olympics, she made two finals, finishing eighth in the 400m and fourth in the 800m, only two seconds off the bronze medal.
Three bronze medals followed at the 2013 world champs, the Commonwealth Games in Glasgow last year brought gold and silver medals in the 400m and 800m respectively, the win in Games record time.
Now this year, and Rio to follow. There's a pattern here. Boyle admits she's something of a late bloomer in a sport where swimmers can often be at their best before turning 25. She puts it down to not flogging herself from the age of 18 to about 23.
"I didn't do the massive miles that often make distance swimmers burn out. I learnt heaps about technique, got my degree [in business from Haas School of Business at the University of California Berkeley] and I think it's given me a lot more years to spend in the sport."
One thing about Boyle shines through. She really loves swimming. This is not a chore for an automaton.
"Lots of days I love swimming so much and don't want to stop. I get to go to so many amazing places and keep really fit for my job. I am so lucky to live this life.
"At some point I might want do something completely different, but right now I'm really enjoying what I'm doing."
She recalled with a grin that "someone came up in a supermarket after the Commonwealth Games and said 'well done'. But I don't think I'm that famous."
Her profile is much lower in New Zealand and in any case it's no big deal in her mind. "I don't do swimming to be famous. I do it because I'm passionate about the sport."
The biggest pleasure out of Kazan was making improvements in her times.
"That's what I strive for and the best feeling I can get from the sport, that I've changed something or grown in some way to improve my performance to a level it's never been before."
Take away Ledecky and we would be talking gold medal contender in Rio. Boyle has studied her action to try to glean a nugget or two of information which could help. She likes the young American, a 10-time world recordholder whom she calls well balanced and very friendly.
"What she's done over the last two years is something we've never seen before and probably won't for a very long time."
How to beat her? "If we knew we would," Boyle quipped. "Obviously I need to keep improving to get closer but I'm not sure if anyone is going to figure that out soon."
The next 10 months of her life are being mapped out. It's not set in stone yet but she's got a fair idea what will be involved. Ten months? At that point the Olympics will be around the corner. Boxes need to have been ticked by then, otherwise more than likely it'll be too late.
You hear athletes talk of withdrawal symptoms if they take a few days away from the track, pool or velodrome. They'll always be wondering what their rivals are up to. Are they getting an edge while you're taking time out?
Boyle has had a few days off and is relaxed because she wants her body to recover and wants to start the next season fresh. But if it was in season, unless she was ill, "I just would not do that".
Perhaps a more pertinent thought she'd have is even when you are training full on, "is someone doing it better?" Right now, in Boyle's field, there's only one person doing it better than her. Ledecky might be beyond her reach, but she has put herself in the best possible position for a big year.
