It's British (probably Commonwealth) English for serious or important. See usage 6: https://en.oxforddictionaries.com/definition/heavy (unfortunately, no dates given on the quotes).
This lists the usage meaning "severe" as from 1849: http://historicalthesaurus.arts.gla.ac.uk/category/?id=15861...
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.
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.