It's a GorTex vent plug.. I'd replace it with a normal non venting solid vent screw plug.. There's a plastic replacement plug that will fit.. I'm not sure if there's a metal version that will fit.. Try and find a socket that will fit and unscrew it.. It should have a bolt looking cover and they shouldn't be tampered with.. Looks like yours has because it's missing.

Where are you?

That' what I thought too, I'm on the Central Coast NSW

I broke the top off of one once and they're easily replaced...just need to make sure you have the right size socket fully over the plug..and take care not to over-torque it on the way in.