Structural changes of annealed Au contacts on semi-insulating GaAs have been observed by conventional positron lifetime-spectroscopy in which a significant fraction of positrons are drifted from a $^{22}\mathrm{Na}$ source to the contact by an electric field. For annealing temperatures below 200 \ifmmode^\circ\else\textdegree\fi{}C it is found that the interface traps positrons into microvoids with a characteristic positron lifetime of 380\ifmmode\pm\else\textpm\fi{}10 ps. For annealing in the range of 300--400 \ifmmode^\circ\else\textdegree\fi{}C a 150\ifmmode\pm\else\textpm\fi{}20 ps additional component appears in the lifetime spectra that is attributed to positrons annihilating from AuGa phases at the interface. The most likely explanation for this sudden onset of the positron component in the AuGa phases is that the changes in the GaAs near surface electron chemical potential, brought about by annealing, affect the interfacial dipole in such a way as to allow a favorable potential barrier for positron penetration into the metallic phase.