This study proposes an online measurement‐based method for n −1 voltage stability assessment on a load area by means of an equivalent system of N + 1 buses introduced by Hu et al. to represent N boundary buses of the load area and the external system. This method utilises real‐time synchro‐phasor measurements on N boundary buses and online state estimation on the load area to analyse n −1 sensitivities for equivalent transfer admittances and power flows by means of a modified line outage distribution factor in order to estimate the voltage stability margin of power transfer to each boundary bus under an anticipated n −1 condition. The method is demonstrated on the Northeast Power Coordinating Council 48‐machine, 140‐bus power system.