Is it true?

[v0031]

More than 80 percent of Australians live within 100 kilometers of the coast making Australia one of the world's most urbanised coastal dwelling populations.

Or should be 100 square kilometers?

[dead0man]

It should not be square kilometers, the original sentence is fine.

[Lexii, harbinger of chaos and sexual anarchy]

square km makes no sense  in that  context

[🦀🎂🦀🎂]

Yes, the coast is a distance (point x to y) so is measured in km. The 'square' part measures area - it is asking how many squares with one kilometre sides could fit in the land mass.

[v0031]

Should it be 1000 kilometers？

[/]

Should it be 1000 kilometers？

No; the "square" simply means you are referring to area instead of length. Perfectly fine

[Associate Justice PiT]

Should it be 1000 kilometers？

     1000 kilometers is a pretty large distance and wouldn't be all that impressive in the context. The 100 kilometers makes sense because the interior of Australia is a vast desert that few people inhabit.

[v0031]

Could anybody draw a picture to make this clear? Thanks.

[Sprouts Farmers Market ✘]

Could anybody draw a picture to make this clear? Thanks.

Wherever you see the coastline, draw a 100 km radius inland.

For visual sake, it's the brown spraypainted area approximately. Just look where all the major cities are.

[v0031]

Thanks, that makes sense.