As an Australian that has worked with American companies for a while, this is a behaviour I’ve had to overcome. If you tell an Australian you’re “not bad” at something then they’ll take that to mean that you’re actually quite good, whereas many Americans will understand it to mean that you’re not good.
Being overly self-congratulatory comes across as this sort of grotesque and unattractive quality. It is even true in the corporate environments I've worked at where you would normally otherwise expect it. Kiwis don't like it and I've seen many foreigners, especially the few Americans that have come over into management roles completely flounder and have almost everyone hate them because of it.
In Canada being modest is quite important… unless it’s a job interview (whether explicit or implicit), in which case it’s the “one time you should really brag about what you’re good at.”
You get some of that in the US too but there are also way more down-to-earth people there who actually want to help, and advice is actually helpful, rather than regurgitations of PG essays and Lean Startup.
Melbourne, on the other hand, is seen as Australia's Portland.
Me: English, living in Australia for >20 years
In general, I find Americans and Australians deemphasize positive things at about the same rate and in the same circumstances, though the exact phrasing used can differ (although "not too bad" is a case where the phrase is shared in both dialects).
However, Australians are more likely to also deemphasize negative things (e.g., "This weather is average" to mean it's bad) whereas Americans are more likely to use sarcasm for negative things (e.g., "This weather is great" to mean it's bad).
I haven't found that to be true. Tooting your own horn is generally looked upon as impolite in many circles. It's pretty common for someone who is outstanding at something to say "yeah, not bad".
And in a very meta way, it can be seen as a flex. Someone who is undeniably the best at something can say "yeah, I'm not bad" with a smirk to show that yeah, they know they are very good.
Now there are some circles in the US where bragging is common - banking, VC, hell even tech. But that's not America.
“That is remarkable” - It is horrifyingly bad
“Have you had your tea?” - Get out of my house
"Tea", meaning 'dinner', I presume.
It was once universal custom in Australia to offer either tea or a beer when someone arrived - rehydration in a hot country is a thing to do.
"Have you finished that drink yet" - depending on inflection it's either an offer of another or a strong cue to hit the road.