Plunders state waterways (5)

I believe the answer is:

sacks

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

'plunders' is the definition.
(I know this)

'state waterways' is the wordplay.
'state' becomes 'SA' (South Australia).
'waterways' becomes 'cks' (I can't justify this - if you can you should believe this answer much more).
'sa'+'cks'='SACKS'

Can you help me to learn more?

(Other definitions for sacks that I've seen before include "Plunders; fires" , "Dismisses; bags" , "Large strong bags" , "rifles" , "Discharges" .)

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

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