Green’s Dictionary of Slang

(N.Z.) a dishonest racing tipster, hired by bookmakers to persuade punters to bet on useless horses.

1905		Truth (Wellington) 24 June 4: [They are] even more despicable than low-down spielers, all because they are not so well dressed as the shady individual ‘guessers’ who ‘live on the game’ [DNZE].
1909		Truth (Wellington) 23 Oct. 7: What is a ‘guesser’? – He is a man who frequents racecourses ‘guessing’ what horses are going to win... They are in touch with the bookmakers, get hold of the various fools who attend, and put them on to ‘soft things’ [DNZE].