Auckland Council believes up to 130,000 fans could be downtown if the All Blacks make it to the Rugby World Cup final on 23 October.
The crowd forecast was issued on Tuesday night, as a casting vote by the Mayor Len Brown secured funding to expand the waterfront fan zone.
Mr Brown's vote ended a deadlock over whether the council should spend up to $1.5 million on the expansion.
Chief executive Doug McKay says the city needs to ensure there is no repeat of the overcrowding on opening night, 9 September.
The new Wynyard Quarter fan zone will open this weekend.