Green’s Dictionary of Slang

(Aus.) a policeman.

1849		Bell’s Life in Sydney 10 Nov.3/2: He forthwith departed for an Indigo, and having succeeded in finding one, he handed Mrs Haney over to his tender care.
1852		Bell’s Life in Sydney 5 June 3/4: The screeches of Mary Ann attracted some of the Indigos, who gave chase and after a hard run they captured Caroline.
1860		Bell’s Life in Sydney 24 Nov. 3/1: Mr Saber thereupon converted himself into an impromptu ‘indigo,’ and consigned his liberal patron to the watchful care of the Druitt Street janitors.