None of the routes posted so far meet the >25,000 mile criteria (even the option listed by the OP).
Here is a routing that is 25,003 miles (according to gcmap:
http://www.gcmap.com/mapui?P=syd-sfo-lax-atl-jnb-syd)
SYD-SFO-ATL-JNB-SYD
SYD-SFO eg. UA 870 Sept. 21, depart 10:30am, arrive Sept. 21 7:10am.
SFO-ATL eg. UA 1720 Sept. 21, depart 10:52am, arrive Sept. 21 6:32pm (also a Frontier flight that would work here)
ATL-JNB eg. DL 200, Sept. 21, depart 8:17pm, arrive Sept. 22 5:50pm
JNB-SYD eg. QF 64 Sept. 22, depart 7:10pm, arrive Sept. 23 3pm.
Total travel time: 52h 30min.