"I didn't take too kindly to suggestions that I was overweight," McDonald said in the Melbourne Herald Sun. "It's just a load of nonsense. But it's something that inspires you to go out there and chuck it back at them.
"My fitness hasn't been up to scratch, but people have got to realise that when you've been out for three to four weeks with a groin injury and you've only played two games at the start of the season before that, you haven't had much game time.
"I can reassure you that everything is fine, my weight is the same as what it was last season when I was scoring goals and I'll continue to work hard to get fitter.
"Hopefully that will see me get back to the top of my game and have me scoring goals for my club and country."
The former Motherwell striker is hoping to feature in Australia's World Cup qualifier against Qatar on Wednesday in Brisbane.
"If called upon I can do a job and it would be nice to finally get my first goal for my country," added McDonald.
"It's been a while in the waiting and I don't really want to wait too much longer."