Recent damaging earthquakes in New Zealand ruptured faults that were not known to be active. We analyzed New Zealand historical moderate‐to‐great magnitude earthquakes since 1845 (Mw 6–8.2) to estimate the level of completeness of earthquake fault sources in the National Seismic Hazard Model (NSHM) and the paleoseismic record. Our analysis assumes that the historical earthquakes are representative of the paleoseismic record and, due to the small number of events (45 Mw≥6), is qualitative. We find that about half of all historical earthquakes Mw≥7.0 ruptured faults that, based on today’s state of knowledge of active‐fault locations, would not have been identified as active prior to the event. The majority of historical events on faults previously not identified as active were Mw<7.3 and either did not displace the ground surface or were located in areas where the rates of erosion or burial exceed fault‐slip rates. Incompleteness of active‐fault sources in the present NSHM is the greatest for earthquake fault sources with long recurrence intervals of ≥10,000 yr. These inferred unidentified active faults will, in many cases, be located in low strain‐rate areas, where they may make an important contribution to the seismic‐hazard budget.