Apparently the Captain entered the aircraft's position West of Cape Town South Africa, some 11000km away.
The ATSB reported that during flight preparation the captain (ATPL, 22,580 total hours) manually copied the gate coordinates of S33°56.77' E151°09.8', visible on a sign at the gate, into the flight management and guidance system (FMGS), however mistype as S33°56.77' E015°19.8' effective putting the aircraft's systems 161nm west of Cape Town (South Africa) into the Atlantic Ocean more than 11,000km from the actual position at the gate in Sydney.
It's a pity a little map doesn't show up to confirm the position entered. He would then have been able to fix the error.