Breathless and his heart pounding, Walker suddenly found himself standing among a room full of strangers, where he slowly gathered his thoughts and regained his composure.
That was about five years ago, but it wasn't until much later the music artist, now 29, realised that he had suffered the first of many anxiety attacks.
"I identified it from watching and listening to others," he said.
"It's like your body goes 'nup, it's not happening' and you just freeze and you can't move.
"I was stressed at the time over deadlines for music and things weren't going good."
But things were about to get a lot worse for the star.
Walker, who splits his time between Sydney, Auckland and the Cook Islands, was diagnosed with stomach cancer in 2017. The disease, which he is now in remission from, nearly killed him. But that was a walk in the park compared to his bouts of anxiety.
"Cancer was the easiest part," Walker said.
"It's the mental stuff that lasts."
News.com.au is this month raising awareness of good mental health as part of its campaign Let's Make Some Noise. We are highlighting the issue of anxiety and its cost to employers, the community, families and sufferers in support of Beyond Blue.
In a given year, some 2.5 million Australians battle a type of anxiety disorder, which is now the most common mental illness in Australia and the top condition that prompts people to visit a GP.
In their lifetimes, a staggering one-in-five people in this country will experience anxiety in a way that has a disruptive impact on relationships or work.
Walker is simultaneously raising money for the long-term mental health rehabilitation of the Christchurch families and donating his entire Spotify income from his new EP. It's a cause particularly close to his heart because of his own experiences with anxiety and depression.
In his darkest hours, of which Walker says there have "been many", he has contemplated suicide. But the support of family members, close friends and mentors has helped pull him back from the brink on each occasion. For Walker, human interaction and the power of words have always proved stronger than the depths of despair.
"Sometimes someone has spoken to me and it's been life and death for me and they didn't even know," he told news.com.au.
"We're humans, we sometimes lose our way, we fail, we fall over.
"We need that human connection to uplift us and speak life into us.
"We just need to feel loved and like we have purpose."
THE DARK SIDE OF FAME
On the outside, Walker seemed to have it all: fame, fortune, a successful music career, and a glamorous jetsetting lifestyle. But in reality, he often felt like he was "dying inside".
Walker, who was named the winner of Australian Idol in 2009, said the demands of being part of an industry "that will chew you up and spit you out a thousands times over" and other pressures of fame had exacerbated his mental health struggles over the years.
"One moment you're at the top of the food chain and the next you're at the bottom scrambling for scraps with nothing," he said.
"The reality is there's constant flying and being away from friends and family, and at the end of the day you're on our own," he said.
"I've realised I just need to do what makes me happy."
Nowadays, when Walker hears that someone feels anxious, he reaches out and offers support – particularly when it comes to young men who, he says, "don't talk because they don't want to be vulnerable". According to him, simply showing up for others can often be the difference between life and death.
"I know what anxiety and depression feels like," he said.
"I'm not a professional (psychologist). I just know what's worked for me and talking about it makes others realise they're not alone before it's too late – whatever too late is.
"I talk about it all the time but it doesn't mean I'm cured.
Anxiety New Zealand Trust offers a free 24/7 National Anxiety Helpline 0800 ANXIETY (0800 269 4389) for anyone experiencing anxiety, panic attacks, OCD, phobias, stress or depression etc or for family / whanau callers. The Anxiety Helpline offers free advice, support and education including breathing and relaxation exercises, distraction techniques and information on accessing mental health and other services.
The Anxiety Helpline 0800 269 4389 (0800 ANXIETY) or visit www.anxiety.org.nz