The cause of the warping is either excess humidity, or in some cases, too heavy strings, or a combination of both.
For the humidity part, you can dehumidify it. If you have access to a dehumidifier, then place it near the dehumidifier for a few hours and the bulging should subside visibly.
If you do not have access to a dehumidifier, you still have to find some way to remove the moisture content from your guitar. So you might want to invest in some Zorb-It packs or whatever equivalent they have in Melbourne. Then subsequently, try not to leave the guitar out in the open when not in use. Have it in your case with dehumidifying packs, etc.
Regarding using too heavy a gauge, I kid you not. As you already know, the strings exert a huge amount of pull on your guitar top. And it contributes to the bulging at the area under the bridge. It's no coincidence that bulging often occurs there.
Generally, .12s on a spruce top is fine. Spruce, being much stiffer, can also take up to .13s, but I personally would not recommend it.
The effects of the string tension on a cedar top would be far more pronounced than on a spruce top, because cedar is a lot softer than spruce. So personally, I'd never put .13s on a cedar top. 12s are as far as I would go.
At the end of day it's all a balancing act between projection, tone and the well-being of your top.
Hope this helps.
