AIR NEW ZEALAND FLT 6 (BOEING B-747, ZK-NZX) WAS INBOUND TO LOS ANEGLES. IT WAS CLEARED TO DESCEND TO 7000 FT ABOUT 26 MI SOUTH OF THE SEAL BEACH VOR. COAST TRACON WAS TRACKING THE 747 & ANOTHER MODE C TARGET, BUT WAS NOT IN RADIO CONTACT WITH THE 2ND (UNKNOWN) ACFT. ATC ISSUED TRAFFIC ADVISORIES TO THE 747 AS THE 2ND ACFT CLOSED ON A HEAD-ON COURSE. THE 747 CREW MADE AN ABRUPT DESCENDING LEFT TURN TO AVOID THE OTHER ACFT, WHICH WAS CONVERVING AT THE SAME ALTITUDE & PASSED WITH APRX 100 METERS HORIZONTAL DISTANCE. THE OTHER ACFT WAS RPRTD TO BE A SMALL TWIN ENG AIRPLANE. TWO FLT ATTENDANTS & ONE PSGR RECEIVED MINOR INJURIES. THE AREA & ALTITUDE WERE IN UNCONTROLLED AIRSPACE.