A New Zealand couple has been questioned and held in Fiji after the mysterious deaths of five members of the same family this week in the Pacific nation.

A one-year-old baby was found alive at the scene of the deaths, believed to be caused by the ingestion of “an alleged substance”, this week.

Husband and wife Nirmal Kumar, 63, and Usha Devi, 54, their daughter Nileshni Kajal, 34, and Ms Kajal’s daughters Sana, 11, and Samara, 8, were all found dead in the Nausori Highlands on Monday without visible injuries.

Fijian police found the bodies when a local spotted the one-year-old, Samaira Kumar, and alerted authorities.

One report said the baby, now in hospital, was found sitting on top of the body of his grandfather.

Nileshni Kajal with daughters Sana and Samara. Inset: Her parents, Nirmal Kumar and Usha Devi. Photo: Twitter/FBC News

Bhijiba Wati, the mother of Ms Devi, travelled to their house on the day of the deaths to help prepare for a pooja, a Hindu prayer ritual.

She told the Fiji Sun that she felt uneasy when she arrived to an empty house, and devastated when police arrived that afternoon to confirm her fears.

“I couldn’t stop crying and asking God why such a gruesome killing has been done to them,” she said.

The family’s car was later discovered at a nearby supermarket.

The deaths have stunned and baffled Fijians.

An autopsy suggested the deaths were due to “an alleged substance ingested by all five victims”, with further tests to come.

According to state-owned outlet FBC News, police detained and questioned a Christchurch man and his wife, who are known to the family. They have not been named or charged.

Police applied in court to detain the couple for longer than the 48 hours permissible under Fijian law, but the application was denied.

A second person of interest who was being spoken to by #Police in relation to the deaths of the five members of a Nadi family, has now been released. #FijiNews #FBCNews #Fiji
More: https://t.co/OOtJxrtxrC pic.twitter.com/GHuj3asfrh

— FBC News Fiji (@FBC_News) August 29, 2019

However, the same report said a “stop departure order”, which would prevent the pair from leaving Fiji, had been enacted.

A Fiji Sun report said extended family of the dead were gathering in the Nadi suburb of Legalega, where the family lived, to grieve and plan the funerals.

Mr Kumar’s brother told FBC News that learning of his brother’s death was “the most devastating message that I ever received in my entire life”.

-with AAP