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And Spark says the issues may not be resolved in time for today's matches and that it will make a call by 1pm as to whether they would also be screened, free, on TVNZ.
Spark's video streaming service cut out for some fans during last night's epic All Blacks match, with images of the game flickering, pixelating, blurring and buffering or losing signal altogether.
One woman said the game was "like watching 80s TV, the quality was so bad", while another said it was like the game had been played on the moon.
The company said this morning the root cause of the problem was still being investigated with Spark's international streaming partners, but it was not related to New Zealand's broadband capacity.
In a further statement at 10am, Spark said it had been working with its partners overnight.
"Our technical team believes the issue is within the international distribution network via which the video stream is passed from our streaming platform located in the USA, through to New Zealand broadband providers. The team are working to confirm this and put a fix in place.
"At this time, it is too early to say whether the issue will be fully resolved in time for today's matches. Unless we confident of this, then we will look to simulcast tonight's matches on TVNZ DUKE to ensure New Zealanders have a good viewing experience.
"We intend to make a decision on whether or not to simulcast by 1pm today, to ensure our customers have adequate time to prepare for today's matches."
Chief executive Jolie Hodson said last night a "small percentage" of customers had experienced "intermittent issues with the video quality".
The streaming provider eventually moved to simulcast the second half of the game live free-to-air on TVNZ's Duke.
The All Blacks beat South Africa 23-13.
Spark CEO Jolie Hodson said the company was very disappointed that some New Zealanders did not get "the experience they deserved" during an important match.
"Making a quick decision to give them an alternative means to watch the All Blacks was the right thing to do. We apologise to all impacted customers and we will be working with our partners to rectify what happened and ensure the rest of the tournament goes well."
Spark customers - many of whom had paid between $60 - $90 for the privilege of streaming games live through Spark Sport for the six-week tournament - took to social media to complain, with some asking for money back.
One customer said they had predicted the "absolute shambles" and called for an apology and full refund - as well as making remaining games free to air.
Another said they had watched the game in "glitchy, blurry quality" only to find out it was free on Duke in the last three minutes.
A Dunedin man asked for a refund during the game but Spark only offered him $9 of the original $90 he paid for a Tournament Pass.
Denzell Wiese Christian sent the Herald screenshots from his chat with a helpdesk assistant at Spark Sport.
He had asked for a refund on his Tournament Pass but was only offered 10 per cent back.
He was initially offered help to troubleshoot his device before he made it clear he had found another way to watch the game. The assistant then ended the conversation, the chat desk screenshots show.
However, another Facebook commenter said she had been offered a complete refund of the $90.
The glitches appeared to hit customers randomly, with some in Auckland with new TVs and fibre getting appalling visuals while other rural customers said their streaming worked fine.
Spark said most of its customers had continued to stream the game, with streaming peaking at 132,000 devices and dropping to 126,000 by the end of the match.
Spark's contingency plan to move games to TVNZ was reported back in February. At the time the Weekend Herald reported Spark had pledged to air games live on TVNZ within five minutes of an outage.
In June last year Spark's managing director Simon Moutter assured Radio Sport listeners the company had "plenty of practice with streaming" and were "technology experts on this stuff".
Earlier this month a survey by Broadband Compare asked Kiwis how they felt about the streaming service taking the Rugby World Cup rights.
Just 11 per cent of those planning to watch the RWC said they were extremely confident the streaming would go well.
Half of Sky customers were not confident, while Spark Sport customers were on the fence.