From the website:

Southbound cross city services will take the following route:
Parnell Square East turn left onto Parnell Street, then right onto Marlboro Street turning onto Eden Quay and then left over O'Connell bridge and then will continue onto normal route.

Northbound cross city services will take the following route:
O' Connell Bridge turning onto Eden Quay, Gardiner Street turning left onto Parnell street, Parnell Square West and then will continue onto normal route.