Green’s Dictionary of Slang

dirt track n.

the anus; thus dirt track rider/specialist, a sodomite.

[US]N. Mailer Why Are We in Vietnam? (1970) 16: I hope at least Ranald has got the taste and sentiment to be putting it in the young lady’s vagina rather than going up her dirt-track.
[Aus]G. Seal Lingo 115: Related usages are to ride the tan track; dirt-track rider; ride the chocolate canyon, and to drill for vegemite.

In phrases

if the river runs red, hit the dirt track [SE road/road n. (1b)]

when a woman is menstruating, opt for anal intercourse.

Duncan-the-disc-jock posting on ‘DJ Schwede’ at [Internet] I don’t rate highly – too commercial and has been played out many many months ago. If the river turns red, hit the dirt track instead!