Yes, it is saying that the coins have been proven to be 1000 years old.
For this example, only "I" and "you" does not follow this. For "I" and "you" use "have been." The other subject pronouns follow this rule:
Has been = singular
Have been = plural
"the Northern Territory" can be thought of as "it"
It has been...
"five coins" can be thought of as "they"
They have been...
I hope this helps