Green’s Dictionary of Slang

1. (N.Z.) a mix of bicarbonate of soda, used illegally in the hope of enhancing a racehorse’s performance; thus as v., to administer such a mixture; thus milkshaking n.

1990		Dominion (Wellington) 2 May 1: ‘Milkshakes’ – the use of stomach tubing to give a horse a mixture believed by some to enhance performance – came to a head early in March when the [Harness Racing] conference issued a statement outlawing the practice on racedays [DNZE].
1991		Eve. Post (Wellington) 22 Feb. 28: Milkshaking is the common term for administering a large dose of bicarbonate of soda [...] mixture to a horse which harness racing officials say enhances stamina [DNZE].
2003		McGill Reed Dict. of N.Z. Sl.

2. (N.Z. teen) fellatio.

2003		McGill Reed Dict. of N.Z. Sl. 85: get a milkshake Experience fellatio. Teen slang.
2016		‘Chelsea G. Summers’ in hazlitt.net 31 Mar. 🌐 ‘Milkshake,’ ‘skull-fuck,’ ‘hummer,’ or ‘head’ all name it; likewise, you might suck a dick—or you might enjoy getting your cock sucked, or both.