I just flew from Singapore to Guangzhou on SQ.

I just flew from Singapore to Guangzhou on SQ. My luggage came out at Guangzhou wrapped in plastic with my bag opened up. It was obviously not like this when I last saw it (locked it before checking it in of course). My backup mobile phone and a small compact camera inside it have been stolen. Pretty sure it was broken into at Changi as it was checked in overnight for the connecting flight. What courses of action can I take to get some form of compensation?

Contact SQ

Make sure you take photos of it and then tell SQ and they will help you.

I don't think the items were stolen. If an airline has gone through the trouble of wrapping your bag, it usually means it was chewed up by the baggage sorting system at Changi. They've collected as many of your belongings as possible, and wrapped it up to keep it together.
Small heavy items such as the camera are usually lost, as they fall out with ease, when the system busts open the bag.
There will be limited compensation from the airline, and you will likely have to claim the rest back from travel insurance. 

I'm pretty sure it was theft as nothing else from my bag is missing. Furthermore, the theives seemed to have added a random foreign langauge book and a pack of cigarette filters into my bag in exchange for my phone & camera. Out of all the places I fly through that this could've happened I would never have thought of it happening in Changi, even more so whilst flying SQ.

Not wanting to be rude, and understanding that you're annoyed by the incovenience, but I think this further supports my assertion it was the sorting system.
Normally, once one bag blocks the system, others get chewed up also as a consequence. I've never known theives to add items. The foreign language book probably came from somebody elses bag, and the bag handlers in Changi, shoved it back in the wrong bag when the two bags were eventually spat out.
Your phone and camera may either be sitting on the floor under the bag sorting system, or were mistakenly thrown into the other persons bag.

Don't get me wrong, baggage handlers have pilfered bags, but in my experience working in airline baggage services, they usually choose unlocked bags. The customer doesnt discover the stolen items until they leave the airport as their bag doesnt look any different. Once home, the airline refuses to pay, as the customer has already left the airport without reporting it.

Thanks for the insight X. I shall follow up with Changi Airport on that avenue of investigation. Here's hoping that is indeed what happened and that my items are hopefully somewhere recoverable at Changi and not in someone elses bag! 

I would ring SQ ASAP followed by Changi Airport Management as if the did actually fallen out then they might have been handed in, as a last resort (and probroably not worth bothering with) you could try the Singapore Police. If none of these work claim it on Travel Insurance. Hope it all ends well. :)