Vanuatu's opposition says it is yet to get a straight answer from the government about why the Foreign Minister travelled to Papua New Guinea last week.
Alfred Carlot's visit coincided with the controversial transit through Port Moresby of a private plane carrying two Vanuatu diplomatic passport holders, Pascal Anh Saken and his brother.
Saken is the owner of the super yacht Phocea which has been detained in Vanuatu since July due to false documentation.
Mr Carlot reportedly met with the Sakens in Moresby.
The Opposition leader Edward Natapei says he believes the claim by Prime Minister Sato Kilman's office that they knew nothing of Mr Carlot's trip is a cover-up.
"We're still in the dark as to why the Minister travelled to Papua New Guinea and met with the two brothers there. It's very murky and we're just asking questions and not getting any answers. So we're just watching very closely to see what develops now."