Competitors have to raise a minimum $1992 for charity but Making Trax is planning to raise more than $100,000 through sponsorship and donations. The money will be put into a trust fund to allow disabled people to experience outdoor adventures.
Mr Williams runs a business - also called Making Trax - which helps others in wheelchairs experience adventure tourism. He admitted the Mongol Rally would be a challenge.
His team also plan to make the adventure more extreme by skydiving in the Swiss Alps, paragliding in the Austrian Alps and rafting in Turkey.
"I want to do bridge swings, bungy jumps - everything I can find on the way that everybody says people in my position can't ... Myself, I don't see it as being hugely dangerous but there are many things that could go wrong and that's the whole point of having an adventure really."
Going from being an active person to someone who has to be pushed in a wheelchair has been a huge learning curve for the Waipara man who wants to change the way society views disabled people.
"I want to prove a point that people look at people so weird if they are in a wheelchair. Nobody comes up to me and talks to me about how beautiful my blue eyes are anymore - they tend to talk about my blimmin' wheel chair any my disability."
The Making Trax team has almost met the trip's cost of $22,000 through new website Spark My Potential - a spin-off of crowdfunding website Givealittle run by the Spark Foundation (formerly Telecom Foundation).