The 35-year-old began at Cape Reinga on September 3, and was hoping to finish by the end of the year, but is now looking at mid-January to finish in time for his brother's wedding at the beginning of February.
As the best man, he would have to cut his trip short if he didn't finish in time, but he would look to continue afterwards if possible.
He said he also lost four days of walking when he hitchhiked to Auckland for his Dad's birthday last weekend.
Mr Lum had set a goal of raising $1000 for the Starship Children's Hospital, and has so far raised over $700.
"I thought while I'm doing it anyway I might as well raise some money," he said.
"It's not a nice thing for kids to have to go through. I'm able to get out there and they can't even run around the play ground."
The isolation of walking alone had made the journey hard for Mr Lum so far, and he said the five days walking along Ninety-mile Beach had been monotonous.
"There was just sea on one side and bush on the other, you could only tell how you were going by the GPS," he said. "You just walk."
The only training Mr Lum did was his everyday job as a carpenter in Perth, walking around wearing a heavy builder's belt.
Along the way, he said, he was spending most nights in a tent, but also appreciated the assistance from locals. "Now and again they might put me up for the night," he said
You could donate towards his cause on givealittle.co.nz/cause/AndrewLum.