I've been seeing stuff posted here and on Facebook and YouTube that the magazine pictured in the photo was already on the floor when the Christchurch mosque shooter arrived. And that's still possible, but it's not the only explanation. If you watch the video, you'll see him tossing weapons, magazines, and even his rack-mount flashlight onto the floor as he goes along. There is a second or three in which the hallway is not visible, as he's turning to shoot into a side room off the entry hall, and it's during that time that you can hear a sound very much like a magazine hitting the floor a few feet away.
Even if that's not the case, it doesn't necessarily follow that the whole thing was staged. He may have had help on the inside, and an agreement not to open any doors, giving his allies a safe way to ride out the storm.
I've also seen an argument that the empty cartridges "disappear" after a few milliseconds as he's shooting, indicating that the whole thing was a CGI, but if you step through frame-by-frame, you'll see that's not the case. Since he's using a Go-Pro type camera with a fisheye lens, things get smaller exponentially with distance, but you can still see the casings as they fall.
Plus, how would you get an entire city's cooperation in a staged event of this magnitude? I say Occam's razor comes down on the side of the event happening pretty much as reported.