Like Martin I have not heard it used in these ways in Australian or British English. However, with regard to the second one you can generally say: "I would knock a few degrees off the oven temperature." You just wouldn't say "down." In most speech, "to knock down" would usually be used in a literal sense to mean "to demolish." e.g. "We knocked down the dividing wall to make room for a new kitchen." However, such is the flexibility and diversity of Australian English that even if you said the last sentence about someone's salary to me I probably would not even notice since from the context it is very clear what you are meaning. Not all phrases and words would fly under the radar like that, but many statements containing slang are improvised and metaphorical so I would probably just assume that in some idiom, region or subculture of Australia that it is a normal expression. That is to say, even though I have never heard that expression before it doesn't sound altogether wrong either to my Aussie ear. Slightly unrelated, but there is an adjective "knockdown" which can be applied to an argument that is irrefutable. e.g. "They were debating but then the man shut his friend up with a knockdown argument."

Hey Nadia, I'm from the UK and I'd say we never use "knock down" in the ways you've described - at least in my region. "We need to knock down these pallets before we can start to take stock of this merchandise" would be something more like: we need to clear / work through these pallets before we can start to take stock of this merchandise" (I'm assuming there were several pallets that needed to be organised or removed before you could check the merchandise). "I would knock down the oven temperature a little bit. You don't want to burn the pie." For me, the more natural way to say this would be: I would drop the oven temperature a little bit. You don't want to burn the pie. "He never stops bragging about how he knocks down six figures a year." There are different ways to say this. More typically you'd say: He never stops bragging about how he makes six figures a year. A more-casual way would be: He never stops bragging about how he bags six figures a year. Hope that helps :)