Like most security measures your padlock deters opportunistic trouble makers.
If somebody is determined to get into a room / building / house etc they will get in.
As an aside we have loads of grief at work about "computer security" which makes me laugh since if somebody wanted the data on our servers a good kick on the door and you would have easy access to everything.....
If somebody REALLY wanted access to your polytunnel they would damage the structure, kick it down, whatever. My little padlock on my shed deters the little chavs from nicking my spade, if they really wanted it they could lift the roof of the shed.
Get a replacement lock, ask for one that's designed to be outside. A lot of the brass type padlocks sold are not suitable for prolonged outside use and start to get stiff quickly.
Hope this helps,
Woppa