A young New Zealander who died in Thailand last month had a highly infectious virus which can spark the heart condition which killed her, tests show.

Sarah Carter, 23, died in Chiang Mai, where she was holidaying with friends Emma Langlands, 23, and Amanda Eliason, 24. All three were seriously ill and suffered vomiting and heart complications but only Ms Carter died.

Four other people - an elderly British couple, an American woman and a Thai woman - also died unexpectedly in Chiang Mai within five weeks and their deaths are being investigated.

But the Dominion Post today reported tests carried out on Ms Carter showed she had echovirus - a highly contagious disease which can cause myocarditis, the heart condition which killed her. The disease is linked to dirty, overcrowded conditions.

The discovery was revealed by Thai doctor Pasakorn Akarasewi, who met New Zealand embassy staff to report the investigation's latest findings on Thursday night.

British couple George and Eileen Everitt died in the Downtown Inn - the same hotel Ms Carter and her friends were staying in - just two weeks later, while Thai woman Waraporn Pungmahisiranon, 47, died in the room next to Ms Carter on February 3, the day before the New Zealanders became ill.

The hotel's air conditioning and facilities have been checked but nothing suspicious found.