If you want to get into the semantics of it, we can:

Always has always had a range of meanings, the most common being "forever" or "every single time"

Another common usage of it is using it to exaggerate sentences.

In this context, it is using definition number 2

Always has been their land means "it has been there land [for a very long time]"

Just like how European land is considered "always European", Aboriginal Australian land is considered "always Aboriginal Australian"

Another reason people say that is the Aboriginal Peoples of Australia have a unique connection to the land, more so than other cultures do to the land they are on

Their religion, lifestyle, presence, culture is uniquely tied directly to the land that they claim.

I'd suggest speaking to an Aboriginal Elder for a better explanation as I'm merely an Australian who only knows the gist of things