I was part of the first response for the Christchurch earthquake providing technical support and co-workers in my old job are trained USAR engineers that were deployed overnight. But most of this information is from the NZ media. We aren't talking high fatalities, but USAR are looking for a couple of missing people and there are buildings that have collapsed, people were also stuck under a vat in a winery and had to be cut out. There are two people dead but not much information is coming out of the most effected area because most communications are down.
The poster used slang to reference the event and someone else was making fun of the slang by taking it literally, completely trivialising the fact that a huge natural disaster occurred which resulted in death, injury, and hundreds of millions if not billions of dollars in damage.
Surfer here from Australia, "heavy" is very common surfer slang used to describe something "serious" or "unpleasant".
It comes from the idea of a "heavy wipe-out", which can quite easily occur while surfing "waves of consequence". Generally good, hollow, barrelling waves contain more water volume and energy, when they break they look and feel "heavier" and while being better rides, have more potential to injure a surfer. The slang has indeed made it's way into everyday usages in NZ and Oz and Hawaii, I guess California also.
I believe the original poster is describing a serious or unpleasant situation, which seems to be the case.
According to their profile, the author of the post is a native German speaker. In German, the same word ("schwer") means both "heavy" and "difficult" or "serious" (in the sense that somebody in such a situation is carrying a heavy burden). The literal translation would be "heavy".
English is a Germanic language, so perhaps in a sense this meaning has always been part of it. As you point out, even if it's not considered completely idiomatic most people would have no trouble interpreting the meaning.
