The Vanuatu chapter of Transparency International says the Ombudsman must investigate claims about poor conditions at Port Vila prison.
The Ombudsman office has received a document, thought to be sent to the from inmates, detailing overcrowding and alleged human rights breaches.
The Ombudsman office says the Vanuatu prison has been a subject of an Ombudsman investigation previously, and as such the Office is prevented by law to re-investigate the same issue.
However, Transparency's President, Marie-Noelle Ferrieux-Patterson, says it's the responsibility of the Ombudsman to look at all complaints it receives.
She says her agency will re-send a copy of the document to his office.
"We shall send him the report and refer the complaint to him and then he has to investigate. There is no two ways, the constitution requires him to investigate."
A new prison is to be built with New Zealand aid assistance but Mrs Ferrieux-Patterson says she intends checking how soon construction will get underway.