The Firth of Thames is a large bay located in the north of the North Island of New Zealand. It is the firth of the rivers Waihou and Piako, the former of which was formerly named the Thames River, and the town of Thames lies on its southeastern coast.
Its original name is Tikapa.
The firth lies at the southern end of the Hauraki Gulf, southeast of the city of Auckland. It occupies a rift valley or graben between the Coromandel Peninsula and Hunua Ranges which continues into the Hauraki Plains to the south.