That's not quite how it works with snails though. They don't just breed 1 snail each, it's more...
Every week
2 becomes 100
100 becomes 50,000
50,000 becomes 250,000
250,000 becomes 12,500,000
12,500,000 becomes 625,000,000
625,000,000 becomes 31,250,000,000
31,250,000,000 becomes 1,562,500,000,000
1,562,500,000,000 becomes 78,125,000,000,000.
So in 2 months you'll have 78 trillion snails. If they get loose that would be enough to completely cover about 7800 square kilometers and weighing about 3,906,250,000 metric tonnes. That is coincidentally almost exactly the same mass as the entire world's production of crude oil and twice the mass of all the fish in the world. All those starving snails would cover any area from roughly Levin to the bottom of the north island.
The huge mass of snails would either devour every living thing and then migrate northwards eating anything and everything they can before reaching the top of the island. Resulting in somewhere around 4 million deaths and which point they all die of starvation.
Or, being aquatic snails, most simply die at the start and leave us with almost 4 billion tonnes of rotting meat which seeps into the oceans, into the air, polluting everything and causing massive amounts of death in the ocean, spreading disease across the entire country and southern hemisphere. Millions or billions of people dead, massive ecological die offs and extinctions around the world, all because someone didn't control their snail infestation.