READ MORE: Orang-utan attack: Animal's mood hard to predict
Last Saturday around 1.30pm, Mr Wickramarachchi's group of hikers saw a female orangutan and her baby come down from trees about 20 m in front of them. The group slowly moved away from the animal to get out of its way, as their guide advised them to.
"We were expecting, because it had a baby, that it wouldn't approach us. All the other ones that had approached us previously, when we moved away they would just keep going on their way," he said.
But the ape continued to move towards the group.
"They're quite big animals and they have long arms and legs. I think it was about a metre away when it just reached over and grabbed my leg." Mr Wickramarachchi said the female orangutan was fully grown, much bigger than him.
"So I couldn't really get away and then it just bit me... kind of clamped on and then I felt it tightening and tightening and that was when I started struggling - when it started hurting."
Mr Wickramarachchi fell to the ground and attempted to kick it with his other leg, but it was not until a guide hit the orangutan from behind, knocking it over, that it let go.
"We all started running and it followed us for a little while, but it didn't keep coming."
The group had to hike for three and a half hours in rain over steep hills to the nearest campsite for medical attention.
He then had to wade chest deep across a river to a raft, which carried the group downstream to the village of Bukit Lawang.
From there he had a three hour drive to the city of Medan, where he received a rabies vaccination at Columbia Asia Hospital. He got his wounds cleaned and left the hospital a few hours later.
Wanting to be in a reliable place in case the bite got infected, he decided to fly back to Auckland, to his "very worried" parents, the next day.
After having additional vaccines and a check up with doctors here, Mr Wickramarachchi said his wounds were healing well.
"In more ways than one I put myself there and I was very unlucky. You can't really blame the orangutan... I really hope nothing happens to it because of this."