Volcanic ash contamination of high voltage (HV) power networks compromises the reliability of society’s electricity supply. Ash-induced insulator flashover is a common problem on transmission networks during explosive eruptions, which is attributed to the high conductivity (σ), low resistivity (ρ) of volcanic ash. However, there have been few studies which have investigated the electrical conductivity of volcanic ash and how it may be influenced by different volcanological and environmental factors. In this study we have used a simple and rapid testing method to measure the influence of ash composition, grain size, soluble salt content, compaction and moisture (water) content on ash conductivity. We also developed physically, chemically and electrically equivalent ash proxies to be used for current and future laboratory experimentation. Results indicate that dry volcanic ash is non-conducting (ρ > 1.56 × 107 Ωm), however, the conductivity of volcanic ash increases abruptly with the adsorption of water. Further increase in conductivity has been observed with increasing soluble salt content and compaction. All grain sizes (<32 μm to 1.4 mm) can exhibit high conductivity values (ρ < 100 Ωm) and therefore have similar potential to cause flashover on HV insulation. The methodology development and results herein represent a benchmark for in-field testing during volcanic crises and for future studies.