Interfacing single photons and electrons is a crucial element in sharing quantum information between remote solid-state qubits. Semiconductor nanowires offer the unique possibility of combining optical quantum dots with avalanche photodiodes, thus enabling the conversion of an incoming single photon into a macroscopic current for efficient electrical detection. Currently, millions of excitation events are required to perform electrical readout of an exciton qubit state1, 6. Here, we demonstrate multiplication of carriers from only a single exciton generated in a quantum dot after tunnelling into a nanowire avalanche photodiode. Owing to the large amplification of both electrons and holes (>104), we reduce by four orders of magnitude the number of excitation events required to electrically detect a single exciton generated in a quantum dot. This work represents a significant step towards achieving single-shot electrical readout and offers a new functionality for on-chip quantum information circuits.