"Three hundred games for the NRL, runs round the footy field, all these big moves ... that's not the real me.
"You know how I know that? 'Cause I don't even know the real me. There's a half of me missing that I haven't found out about."
Marshall went on to reveal that growing up in Whakatāne in small-town New Zealand, he never knew his real father. He was born when his mum was just 15 years old.
"I don't know my culture, I don't know my nationality. Who am I?
"I have this vivid memory of a seven-year-old little Māori boy in his lounge room holding a butter knife, scared, sitting back in the dark in the corner so no one could come from behind and get me."
"All I wanted was someone to save me, someone to tuck me in, someone to love me. All I wanted was a dad. In the playground at school when I was a kid, the kids would tease me: 'ha, you don't know your dad, you don't know your dad'. And I would sit there in the playground crying because I felt different to everyone else."
One moment that stayed with him for years was when he asked his mum who his dad was, he said.
"I'll never forget the look on her face. It was the look of fear, worry, and it actually made me scared and I never asked her again. I still don't know to this day," Marshall shared.
He then told the judges that in the docuseries, he wants to take viewers on a journey to find out who his real father is - and who he is himself.
Marshall appeared alongside radio stars, models, singers and other sports stars on The Celebrity Apprentice Australia, a spin on the US Apprentice format which sees celebrities compete to raise money for their chosen charity.
The four-time NRL All Star announced his retirement in October last year after 19 seasons in a career that saw him suit up for the Wests Tigers, St George Illawarra Dragons, Brisbane Broncos and South Sydney Rabbitohs, as well as a short stint for the Blues.
Marshall, also the longest-serving Kiwis captain, was made a Companion of the New Zealand Order of Merit in this year's Queen's Birthday Honours for services to rugby league.