Kate Weatherly wooshes down a trail at Riverhead Forest west of Auckland, hitting jumps aggressively and controlling her Devinci Wilson mountain bike like it's an extension of her body.

Fresh from winning the elite women's division at the national championships near Wanaka in February, the 20-year-old is back into a busy routine of university study, training and part-time work at a Kumeu bike shop.

Weatherly's sudden arrival on the women's downhill scene in January came as a surprise to some in mountain biking - until the end of last year she'd been known as Anton and raced men.

She'd quietly let a few of the other women know she was making the switch and they seemed supportive. But when she won an event in Rotorua by more than 30 seconds, it set off a firestorm of online discussion and calls for her to be excluded.

She had an unfair advantage because she developed as a male, people said on bike forums. She could "ruin" the careers of other riders and discourage women from taking up the sport, some complained.

Weatherly had previously competed in the men's open division, a grade down from elite, and prior to that the novice men's and under 19 sections.

She hadn't exactly set the world on fire - usually finishing mid-pack. (Downhill riders race the clock, rather than each other).

No-one knew that she wasn't just competing against other riders, but her own body - the hormone blockers she'd been taking since 17 had reduced her testosterone levels to below those of most women.

"I was losing strength and losing speed on tracks even though I was training and riding really hard," she says. "It was demoralising - you're trying to push yourself as far as you can but you're going slower and slower."

But she didn't want to come out as transgender, fearing hateful comments in a male-dominated sport.

"So I thought, 'I'll just keep racing in the men's field at a disadvantage'.”

Her times began improving again - although she had to train harder than ever. She figured she'd look into switching to the women's category for the 2018/19 season, but her plans were up-ended at the very first race of this season, in Rotorua last September.

"Someone found out and went around telling people and kind of outed me, which was quite traumatic.”

However, she was pleasantly surprised by the support she received.

"I thought, 'oh well, everybody knows, I might as well' [switch]."

She got in touch with Mountain Bike NZ and Cycling NZ, which has a transgender policy requiring that transgender people be treated "as belonging to the sex in which they present...unless this might give [them] an unfair advantage".

Like many sporting federations around the world, Cycling NZ has adopted guidelines issued by the International Olympic Committee in 2015 , requiring transgender female athletes to prove their testosterone level in serum has been below 10 nmol/L (nanomoles per litre) for at least 12 months prior to first competition.

The average testosterone level for young men is 24.2 nmol/L and for women around 0.6 nmol/L.

Weatherly was able to provide blood tests showing her levels were in the 0.1-0.9 range, a result of having had injections of the drug Lucrin - used as a treatment for prostate cancer and essentially a form of chemical castration - when she started transitioning at 17.