Aussie holidaymakers heading to a picturesque island have been warned to cover up when they get there out of respect for the local culture.

Carnival Splendour, which left Sydney on June 2, sent a letter outlining the dress code on the island of Lifou, New Caledonia - their first port of call.

'To observe the local customs of the island, a respectful dress code is essential,' ship master Eduardo Ferrone wrote.

'Swimwear should only be worn at the beach or by a pool and should be very modest in nature. 

'G-strings, thongs, monokinis or mankinis, should not be worn (even on the beach) and topless sunbathing is also not permitted at any time.'

The letter went on to tell passengers that while exploring Lifou, they must wear casual clothing at all times.

'As a visitor to the island, please respect the wishes of local residents,' Ferrone wrote.

'Non-compliance could severely restrict or compromise our ability to keep the island as part of our future Carnival holiday itineraries.'

Lifou is the largest of the Loyalty Islands in New Caledonia, and is renowned for its powder-white beaches, limestone cliffs, and rich Melanesian culture.

It is a premier South Pacific cruise stop and is a 40-minute flight from Nouméa.

Lifou Island boasts a deeply rooted Kanak culture governed by ancient chiefdoms and a strict social hierarchy.

Local practices for the highly religious society call for conservative attire.

Visitors should wear modest clothing covering their shoulders and knees as swimwear is prohibited outside of the beach.

Carnival Cruise's website warns travellers: 'Because Lifou is a tribal island, passengers must wear modest, conservative clothing. Revealing swimwear is strictly prohibited by both the cruise line and local customs.'

Despite all the warnings, not all tourists are adhering to the dress code.

One person told Cruise Passenger: 'They did not cover up. It was rude and disrespectful and Carnival should not have allowed them off the ship dressed like that. It will ruin it for everyone if people don't respect the culture.'

One traveller suggested Carnival should do a better job at notifying guests before boarding, to ensure they have alternative swimwear packed.

'We were given a letter with the rules but I would bet that the young woman wearing the g-strings wouldn't have brought any other swimmers. They need to tell people prior to sailing. I know my daughter only owns g-strings and would have needed to buy a pair of swimmers just for the trip,' they said.

Some online encouraged travellers to defy the dress code.

'Wear what you want,' one Reddit user said.

Others said cruisers who want to wear revealing clothing should stay on the ship.

'Once you get off the ship you are no longer "on a cruise", so respect the local customs, cover up,' one person wrote online.

'Lifou is a tribal island home to the Kanak Indigenous people, whose culture is based on respect and is highly religious,' another person wrote.

'Bottom line, bring a modest one-piece swimsuit and a rashie or cover-up. Respect the local tradition. Their country, their rules.'

Carnival recently reminded cruisers visiting Jamaica not to wear camouflage clothing when they leave the ship because local laws restrict that attire exclusively to military and police.

A Carnival spokesperson told Daily Mail: 'Carnival Cruise Line respects the local cultures of the destinations we visit in the South Pacific and works to help our guests understand and appreciate local customs.'

'For many years when visiting Lifou, New Caledonia, we have shared guidance with guests about appropriate dress, including avoiding swimwear in public areas, as a sign of respect for the local community,' the spokesperson said. 

'Travel plays an important role in building understanding between cultures, and we aim to support our guests to engage with the places they visit in a thoughtful way.'