Mr Key had been booked to attend the Italy v Ireland game at Otago Stadium on Sunday, but will now watch the Auckland-based Warriors play in the NRL grand final the same day.
"He believes it important that he show support for both rugby codes, hence his commitment to attend the grand final," a spokesman for the PM said yesterday.
He rejected any suggestion the move was a snub to the South Island.
"Suggestions of a 'snub' to the South Island are silly," he said, saying Mr Key planned to watch a quarter-final from a Christchurch fan zone.
University of Otago politics lecturer Dr Bryce Edwards said New Zealand prime ministers had little choice but to support sport, particularly rugby.
However, Mr Key's decision to watch the Warriors in Sydney rather than a World Cup match in Dunedin was "entirely a calculated decision".
The Auckland vote was crucial in determining the make-up of the Government, with the Warriors game offering Mr Key better exposure to more voters.
Rugby World Cup Minister Murray McCully also had not attended any RWC games in the South Island.
"The minister has been booked to attend some South Island games but the evolving nature of the tournament has meant he hasn't been able to attend," a spokesman said.
The minister had taken a close interest in the development of Otago Stadium, and "he can assure the ODT that there is nothing he would like to do more than go to a match at Dunedin".
One minister who has attended a game at Dunedin's new stadium is deputy prime minister and Clutha Southland MP Bill English, who saw Dunedin's opening clash, between Argentina and England.
Mr English also attended the Scotland v Romania clash in Invercargill.
