The following tabulation translates the code displayed on the two line matrix screen on the top of the automatic entry/exit gates at CityRail stations. When a ticket is rejected (ticket is returned, but does not activate gate), the screen displays an error message which looks like: 
INVALID 00
SEE ATTENDANT
where "00" is replaced by a number between 01 and 28 matching to one of the error reasons below. Otherwise the display simply reads
TICKET CAPTURED
for exit at the end of a journey on single and return type tickets or 
TAKE TICKET
for station entry on all ticket types, and exit for periodical tickets and first journey on return tickets.

01Invalid Encryption Code
02Invalid Mode
03Not Available On Rail
04Full Fare Tickets Not Allowed
05Concession Not Allowed
06Used Before Valid Start Date
07Expired
08Not On Route
09Passed Back
10Not Valid Entry Station
11Return Ticket Expired
12Not Valid For Exit
13Hotlisted
14Ticket Already Used For Enter
15Previously Hotlisted
16CityHopper Expired
17Entry Station Outside CBD
18CityHopper Not Valid For Exit
19Data Integrity Error
20Group Not Allowed
21Platform Ticket Not Valid
22Single Cannot Exit At Origin
23Forward Journey Already Used On A Return Ticket
24Return Journey Already Used On A Return Ticket
25Verify Failed
26THM Read Data Late
27Co-ordinator Read Data Late
28Co-ordinator Write Data Late