This paper reports the first observation of a giant kōkopu (Galaxias argenteus) spawning site in a tidal waterway and illustrates the usefulness of DNA barcoding for opportunistic discoveries that in this case involved the collection of a single egg. We describe key characteristics of the spawning site and compare them with the only other recorded giant kōkopu spawning location which was found in a much different environmental context far inland. This is also a novel observation of giant kōkopu and īnanga (G. maculatus) spawning on the same section of riverbank. Eggs from the two species occupied the same vertical elevation band, which together the consideration of tidal heights and the egg development stage suggest that the spawning event was triggered by a tidal cue. We discuss the implications of more widespread giant kōkopu spawning in tidal waterways, including historical perspectives contributing to their decline and the potential to reverse these trends.