The Townsend first ionisation coefficient alpha /p0, electron attachment coefficient eta /p0, effective ionisation coefficient alpha /p0 (= alpha /p0- eta /p0) and total secondary coefficient gamma T for SF6 and helium mixtures have been measured by the steady-state Townsend method for 54<or=E/p0<or=215 V cm-1 Torr-1 (153<or=E/N<or=608 Td), together with the limiting E/p0, (E/p0)lim, which represents E/p0 for alpha /p0= eta /p0. The results show that the curves of alpha /p0 and alpha /p0 against the fractional SF6 partial pressure k at a fixed E/p0 have a maximum for E/p0>or=86 V cm-1 Torr-1. However, the value of alpha /p0 and alpha /p0 varies approximately linearly with k for E/p0<or=54. Tests at E/p0=107 and 161 V cm-1 Torr-1 for k near the peak did not show pressure dependence in the alpha /p0 value. The eta /p0 decreases with increasing E/p0 at fixed k, and increases with increasing k at fixed E/p0, for k<80% for which eta /p0 is deduced in the present work. It is also found that the gamma T for the mixtures decreases rapidly with k at fixed E/p0. The present value of (E/p0)lim for SF6 and helium mixtures increases approximately linearly with k, in contrast to SF6 and N2 mixtures with which it increases rapidly at small values of k. Possible explanations for the variation of alpha /p0 and (E/p0)lim with k, as mentioned above, are given.