Green’s Dictionary of Slang

(N.Z. gay) an underground public lavatory used for soliciting.

2007		W. Ings ‘A Convenient Exchange’ in Public Space 3 14: Two of the most generic code names for underground bogs at this time were catacombs or grottos [...] Another of New Zealand’s first underground ‘grottos’ still operates in Durham St West in Auckland [...] This bog called in code The Star of David or The Jewish Grotto was named in reference to large cast iron screens that feature a Star of David motif at its entrance.
2010		W. Ings ‘Trolling the Beat to Working the Soob’ in Int’l Jrnl Lexicog. 23:1 63: As underground toilets began to emerge [...] in the bog trade they were generically termed catacombs or grottos.