But the 20-year-old told the Herald last night he had not been taking diuretics, which used to be commonly used to help jockeys drop weight suddenly.
McDonald flew into Hong Kong on the day of the races and says he commonly takes pills after flying.
"I take pills which bring down swelling in my hands and feet which I get when flying long distance," he said.
"I took one that day and apparently, as I have found out, they have a small amount of diuretic in them.
"The amount detected in the sample was very, very small but the testing machines they use up here are so good it showed up.
"I am disappointed but I will cop the suspension and move on."
But McDonald admits he has had minor weight issues, which usually force him to lose 2kg every raceday.
"My normal walking weight is 57kg but on raceday I have to get down to 55kg usually, sometimes 54kg.
"But I don't use pills to do that."
So can the man touted as potentially one of the greatest jockeys produced in New Zealand continue that daily fight against his body?
"I am confident I can," says McDonald. "Yes, I have occasionally struggled with my weight but I am 20 now and have stopped growing so it has stabilised.
"So it is up to me to exercise and eat right and keep it under control and I am dedicated to doing that.
"So while I will have to work at it, this is my career and I want to stay riding for a long, long time."
McDonald will return to New Zealand on Monday and stay busy riding trackwork and at trials to keep in shape during his suspension.
While the suspension will get the tongues wagging inside racing's incessant rumour machine, don't expect McDonald's popularity with trainers to take even the slightest dent.
He rode 121 winners in New Zealand this season from just 524 starters and his $3.2 million in stakes earned is over $1 million more than any other jockey.
But that only scratches the surface as he exploded on to the Australian scene with a solid spring eclipsed by a golden group one autumn.
And the magical season was capped by winning the Champions Mile in Hong Kong with Xtension on May 6, a victory which has him knocking on the door of international racing fame.
He is now being headhunted by several leading Australian trainers as well as being keenly sought in Hong Kong, where racing experts said the setback would have little effect on his popularity as long as it is not repeated.
"People recognise his talent and as long as he keeps his weight under control it won't be an issue for trainers up here," a key Hong Kong racing insider said.
McDonald is likely to ride in New Zealand sparingly over the spring and summer but has committed to at least some of our major carnivals.
