The inner-city south stations on the Beenleigh/Robina/Cleveland lines up to Park Road Station are every 15 mins in off-peak.Well you can thank Cityrail's "improved timetable" for that. Hopefully when the new carriages and lines come on stream things might improve. But all of the major destinations are served by frequent services (every 5 to 15 minutes).
It's not like Brisbane where the frequencies drop to half hourly about 5 stations from the CBD on all lines except the Ipswich and Caboolture lines.
Also as for the Ipswich/Shorncliffe/Caboolture. 15 min off-peak only operates between Northgate - Central - Corinda. In the outer suburbs + other lines except for the North Coast line, off-peak drops to half-hourly.
The only suburban exceptions is hourly for Doomben line (similar to Carlingford line in Sydney, but all Doomben services are through to the City) + Tennyson Line (Brisbane's equiv of the Cumberland line in Sydney)