Green’s Dictionary of Slang

(Aus.) a £5 note, a $5 bill.

1894		Truth (Sydney) 14 Jan. 6/6: Well, I get my cut out of it, two beautiful spinikers (i.e. £5 notes).
1905		Sun. Times (Perth) 17 Sept. 4/8: He took eleven pounds out of the [two-up] school. He put a five-pound note in his back pocket and went down to give the remaining six a chance. He promptly lost that, and then took out the ‘spinnaker’ and promptly did that in.
1955		N. Pulliam I Travelled a Lonely Land (1957) 89: I’ll bet the first Aussie taker a couple of spinnakers the Snowy Mountains dream comes true.