In a statement, an Optus spokesman said that "an extremely high number of viewers logging into our platforms just before kick-off causing some systems to overload".
"Some Optus Sport customers experienced a disruption to their 2018 FIFA World Cup viewing experience on Friday night," the statement read.
"While most customers on most platforms were unaffected, we apologise unreservedly to those customers who were, and are continuing to investigate the cause of these systems issues."
Optus received a tirade of abuse online, with former Socceroo Robbie Slater - a current Fox Sports presenter - joining in on the action.
TVNZ will screen seven Rugby World Cup matches live – including the tournament's opening match and the final – the same number offered free-to-air in 2015.
A yet-to-be-confirmed number of delayed matches will also screen. There will be no ads during live game time, TVNZ said.
Rugby fans will be able to stream matches and related content live, or on-demand, over their home broadband or mobile connection.
The service will offer both free and paid content to everyone, not just Spark customers.
Spark has already released a statement saying it's mindful "some people may still not have adequate coverage to stream the matches at home" and that the company is looking at a range of options to ensure the content reaches a broad audience.
Spark has still not announced what those measures might involve, but the pressure will be on to ensure it delivers effective options for what will be the New Zealand's first predominantly digital Rugby World Cup.
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