The finding eases blame on ship owners who were accused of being part of a sabotage plot after a dragging anchor left Tonga with drastically limited internet and phone services for 12 days.
The incident on 24 January sparked widespread panic and cost Tonga an estimated $US200,000.
Police launched an investigation in February to assess whether the damage to the cable was deliberate or accidental. A report was due back in July but police have refused to release the document or comment on the investigation.
A police spokesperson could not be reached for comment on Wednesday.
However, a May briefing prepared by New Zealand's Ministry of Foreign Affairs and Trade (MFAT) said the report "concludes there was no foul play and a ship had dragged its anchor". The briefing, which was obtained under the Official Information Act, did not include a copy of the report.
The owners of the ship - identified by Tonga as the Duzgit Venture - have said they were assisting investigations.
RNZ Pacific reported in August that the Turkey-based Duzgit had previously admitted it could have "trapped a cable with its anchor" in Tonga on the day the internet cable was cut. Duzgit could not immediately be reached to comment for this story.
The director of Tonga Cable, which manages Tonga's internet cable, Paula Piukala told RNZ Pacific in July that the ship's actions were "intentional" and "clearly sabotage" by a powerful but unknown actor, but has not provided any evidence to support his claims.
Mr Piukala did not respond to a request for comment on Wednesday.
Tonga's Police Minister, Mateni Tapueluelu, has said Tonga's government would seek costs against anyone responsible for the cable cut. Mr Tapueluelu did not respond to requests for comment.
"It is likely the government will seek compensation from the shipping company," the May MFAT briefing said.
As of September, the Duzgit Venture was still making oil shipments to Tonga, according to its local agent, Dateline Shipping.