Here in Australia (and in most of the English-speaking world), there are two separate meanings for narc and nark. The former is an abbreviation of narcotics officer. And the latter meaning a police informer. While it is interesting that they both involve law enforcement, there is not necessarily a correlation between the two coinages.
While there is a trend among some people to use the spellings interchangeably, both for the meaning of police informer, the most common spelling is in fact, nark.
Of course, like all good colloquialisms, this has enough meanings that it can easily be transplanted into everyday conversation as well. To be narky is to be irritable or bad-tempered. A nark can be a scolding, complaining person, someone who is always interfering and spoiling the pleasure of others or a spoilsport. So its uses can be quite a bit wider than simply a police informer.
Each week, we have a look at a slang word from Australian English. You can see other Aussie Word of the Week posts from the Macquarie Dictionary here.