Mr Williams said there were two reasons to support the Live Below the Line challenge.
"To try and understand what it would be like to live in poverty. It's based on the idea of living on $2.85 a day for all your food which represents the international poverty line.
"That reflects on what people have to live on, not just their food but all their basic living costs.
"The other part is to get people to sponsor you and to fundraise for the cause. There are a whole bunch of different causes but we have chosen to support TearFund and their work to free young girls from human trafficking and sexual slavery."
Mr Williams said porridge, sandwiches and tinned spaghetti were his main meal options for the five-day challenge.
"Because they are cheap and filling. I like variety in food and to snack a lot but you don't get that with this because the budget doesn't allow for it.
"You just have to lump it for the cause and get flavour where you can, half an apple here or there to cut through some of the blandness of the carbs.
"It is hard sometimes because you are just eating bland food, and less often than you would normally eat."
Water was free, so he and his group could drink as much as they wanted, he said.
The group started the challenge on Monday, and it will end on Friday.
On Monday, Mr Williams had half a cup of porridge and half an apple for breakfast.
For lunch he had a peanut butter sandwich with another half an apple.
Dinner was tinned spaghetti and toast.
Breakfast and lunch would be the same each day while two dinners would be swapped out for potato, sausage and peas.
"Meat is pretty expensive so you only get a little bit so often."
To support St Peters Youth, click here.
TEARFUND
TearFund is a faith-based aid and development organisation. They combine the inventiveness of our local team with international partners to unlock potential and bring about real change in the lives of people most in need.
Learn more at tearfund.org.nz