Wanting to give kids and parents something fun to do outside during school holidays, McCarthy was inspired to do something around the lake.
"We live around the lake and often walk around it. We wanted to something with the lake and we work with City Mission. We donate bits and pieces every week, but we wanted to do something big."
McCarthy has set it to one free ice-cream, per rock, per person each day so everyone has a fair chance at winning.
She has been blown away by the community's response.
"I had thought I would hide five or 10 a day, but I think I'm going to be hiding them all numerous times a day. I can imagine I'll be doing a few laps around the lake a couple of times a day."
The initiative has been a good way to get the community involved in helping out a very worthy charity, said McCarthy.
"We didn't want to put a dollar figure at it. But we have a list from them what they need and ordering all in bulk for whatever they need.
"We wanted it to be a contribution that made a difference. When we drop things off, it's a scratch of the surface for what they go through.
"Who would have thought it would be popular."