Williams, who was born in Queenstown and lived in the resort for her first four years, said the trio saw the ride as a "bucket list item".
"We like doing challenges."
Wanting to give the ride a fundraising purpose, they found Oscar's family on crowdfunding website Givealittle in April.
As well as Oscar being "bloody cute", his family was struggling to raise money for a rare operation to relieve his condition and to cover the cost of ongoing therapy.
The $100,000, operation, which is not publicly funded in New Zealand, was successfully carried out in London in September.
They hoped to raise $7000 to cover the cost of a specialised treadmill that would aid his rehabilitation, she said.
They met Oscar for the first time on December 20 on their way south and would see him again on their drive home.
Williams said they were amazed by the goodwill of people they encountered on their journey.
"The biggest thing we've learned is how kind and generous people are.
"We've only paid for accommodation once in the 25 nights so far."
Typical of that generosity was the free campsite they were given at Robrosa Station by the organisers of the Rhythm & Alps music festival, where they spent New Year's Eve.
They expect to end their ride in Bluff today.