As Australia is the main backblock? (7)

I believe the answer is:

seagirt

I'm a little stuck... Click here to teach me more about this clue!

'as australia is' is the definition.
I can't judge whether this definition defines the answer.

'main backblock?' is the wordplay.
'main' becomes 'sea' (the main is an old name for the sea).
'backblock?' becomes 'girt' (I am not sure about this - if you are sure you should believe this answer much more).
'sea'+'girt'='SEAGIRT'

'the' acts as a link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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