"can someone please explain to me, how AGL can enter into a...

"can someone please explain to me, how AGL can enter into a contract with BG/QGC to store and supply gas to QGC, from a storage facility which they don't even own????"

They can do however they choose; it's only a risk to the buyer about being unable to deliver on that part of the contract - but being AGL and the size of that part of the deal, it's a very low risk.

It's then up to AGL how to deliver on that; they could build their own, lease facilities from someone else, or buy them outright as they're trying to do in this case.

Definitely nothing wrong with that...