In the extinct South Island dialects, "ng" merged with "k" in many regions. Thus "Kāi Tahu" and "Ngāi Tahu" are variations in the name of the same iwi (the latter form is the one used in acts of Parliament). Since 2000, the government has altered the official names of several southern place names to the southern dialect forms by replacing "ng" with "k". New Zealand's highest mountain, known for centuries as "Aoraki" in southern Māori dialects that merge "ng" with "k", and as "Aorangi" by other Māori, was later named "Mount Cook", in honour of Captain Cook. Now its sole official name is "Aoraki / Mount Cook", which favours the local dialect form. Similarly, the Māori name for Stewart Island, "Rakiura", is cognate with the name of the Canterbury town of Rangiora. Likewise, Dunedin's main research library, the Hocken Collections, has the name "Uare Taoka o Hākena" rather than the northern (standard) "Te Whare Taonga o Hākena". Maarire Goodall and George Griffiths say there is also a voicing of "k" to "g" – this is why the region of Otago (southern dialect) and the settlement it is named after – Otakou (standard Māori) – vary in spelling (the pronunciation of the latter having changed over time to accommodate the northern spelling). Westland's Waitangitaona River became two distinct rivers after an avulsion, each named in a differing dialect. While the northern river was named the "Waitangitāhuna River", the southern river became the "Waitakitāhuna-ki-te-Toka", using the more usual southern spelling ("ki-te-Toka", "of the south", would be rendered "ki-te-Tonga" in standard Māori).