The firefighters got there quickly but Harrison had started to make his way back to the edge after losing interest in Alberta the cat.
However, it was a good job they were there because the infant managed to get himself stuck on a joist and had started crying.
"I was panicking because I'm pregnant so I wasn't about to crawl under there."
One of the five firefighters from the Titirangi station had no qualms about getting on his belly and slithering under to reach Harrison. They tried to pull him out with a blanket but that didn't work and Harrison started getting very upset.
"They asked me whether he has any favourite toys but he likes raisins, because he's really greedy so I gave him a packet of raisins," Mrs Cowie told the Herald.
With Harrison munching away, the adults hatched a plan to get him out.
"One of them asked me if it's a new deck and whether they could pull up some of the boards. I said, 'Of course you can pull it up! My baby's stuck under there!"
Using hand tools, they pried about three pieces of freshly nailed down timber and the man under the deck passed the young explorer out to his mother. As a precaution, a St John ambulance paramedic checked Harrison over but he was fine.
"He stopped crying immediately. Then he saw the fire truck, he loves fire trucks, so he got really excited to see it. The firefighters were great, they gave him some stickers and even put the hose on for him - he loved it."
After such an eventful day, Harrison fell straight to sleep last night. Meanwhile, Mrs Cowie is already planning how to tell the story of the time he got stuck under the deck at his 21st.