In the emotional clip, Stan talks about his health battles and the impact it's had on his family.
'It's been hard for my mum, she cries every day and she blames herself too,' he said.
Voicing his fears before his surgery, he said: 'What if there's complications and I die?'
The New Zealand Herald reported last month that it believes Stan has 'had his stomach removed' due to the condition.
The singer is believed to have a rare condition that can cause cancer, due to a mutation in the CDH1 gene.
Walker was forced to respond to fans when an image of himself looking incredible gaunt made headlines, writing: 'I bet half the people thinking I was on crack or whatever y'all were saying are feel pretty dumb right now.
'Ah well, we are all know it alls sometimes & just make up stories 😂😂😂 its all good. This is bigger than all the talkers talking. It's bigger than me. Its bigger than whats happened & is happening to me. There is always someone worse off going through alot worse. All im gonna say is that im blessed to be alive & well. God is good & ive been blessed with a back bone of whanau & friends that go through the highs & the lowest of lows with me.'
The documentary will air this Sunday night in New Zealand.