Australian ballot vs secret ballot

Australian ballot

noun
  • The secret ballot system of voting in which voting is done in private on a state-issued ballot, and ballots incorrectly marked are set aside as invalid. 

secret ballot

noun
  • The voting system involving ballots available only at official polling places, prepared at public expense, containing the names of all candidates, and marked in secret at the polling places. 

How often have the words Australian ballot and secret ballot occurred in a corpus of books? (source: Google Ngram Viewer )