De Groot was shown a yellow card for making contact with the head of Adriaan Booysen in the last 10 minutes of New Zealand's 71-3 win at Stadium de Toulouse but it was upgraded to a red after being reviewed by the TMO.
New Zealand coach Ian Foster had suggested the All Blacks would argue that the challenge started as shoulder-to-shoulder contact and therefore was not a red card offence.
The independent panel decided, however, that De Groot's failure to wrap his arms around his opponent meant the tackle was always illegal and the head contact meant it was worthy of a six-match ban.
The ban was halved for De Groot's previous good behaviour and will be further reduced by one match if he attends a World Rugby coaching intervention programme.
The loosehead will therefore miss the clash with Italy in Lyon on September 29 and New Zealand's final pool match against Uruguay in the same city a week later before becoming available for the quarter-finals.
With tighthead Tyrel Lomax still recovering from a nasty gash to his leg sustained in a warm-up match against South Africa, New Zealand look likely to have to qualify for the knockout stages without their first-choice props.
De Groot has 48 hours to appeal the decision.
