"Due to a small percentage of our customers experiencing streaming quality issues when watching Spark Sport on certain devices, a decision has been taken to switch live play over to TVNZ Duke," a Spark Sport statement said.
"The New Zealand v South Africa match is now playing live on both TVNZ DUKE and Spark Sport."
"For those customers who are experiencing a poor stream via Spark Sport, turn your TV to channel 13 or DUKE on Sky channel 23. This match is not available through the TVNZ website or TVNZ OnDemand app," the statement said.
Spark's CEO, Jolie Hodson, tweeted her disappointment.
The All Blacks beat South Africa 23-13.
In a statement following the All Blacks' victory, Spark said: "Midway through the first half we identified that the quality of the video stream was fluctuating for some customers. This meant that customers experienced brief, intermittent periods of reduced video quality such as pixilation and buffering."
Spark said it was "uncomfortable" with the experience.
"As we always said we would, we moved quickly to provide them with an alternative means of watching the match. We had prior established procedures with our partner TVNZ to enable live, free-to-air coverage at short notice."
The call was made to simulcast the game on Duke just before halftime.
"The video streaming quality issue improved shortly afterwards and from midway through the second half streaming across all devices appeared stable once again.
"The majority of customers continued to watch via Spark Sport and did not experience video quality issues. The service peaked at 132,000 concurrent streams and by the end of the match had dropped to 126,000."
Spark said the earlier two games - Australia's win over Fiji and the tight French win over Argentina - did not experience the same issues.
"Although some customers needed help from our care teams at times during the day, for the most part these related to isolated device issues and in-home set up.
"The root cause of the video streaming issue is still being investigated with our international streaming partners. However, we can confirm that the issue was not related to New Zealand broadband capacity, with the demand for this game well within our operating thresholds. It was not confined to customers of any particular broadband provider."
Hodson said: "We will be working with our partners to rectify what happened and ensure the rest of the tournament goes well."
In an earlier media briefing issued after fulltime of the first of three huge games tonight - the Wallabies versus Fiji - Spark said about 44,000 subscribers were online for kickoff in the clash; which was won 39-21 by the Wallabies after an early scare from Fiji.
But by fulltime the number of individual streamers had almost reached 88,000.
"Tonight's numbers so far are significantly more than last night's tournament opening match between Japan and Russia," a statement said. "Throughout the game, the Spark Sport streaming service worked well from a platform and broadband network perspective.
"As we always knew, today would be a very busy day for Spark Sport. We've substantially increased our Spark Sport care teams to help our customers with their in-home set up and streaming experience.
"So far today, we've supported approximately 10,000 customers through phone calls, online chat and Facebook and Twitter interactions to get their Spark Sport working as best as possible, based on their in-home set up and devices."
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