Greta and Arno were given limoncello after getting engaged. It killed them.

Greta Otteson and her fiancé, Arno Els, were recently engaged and deeply in love. The pair had moved to Vietnam last year, taking out a lease on the red-roofed Silverbell Villa, a nine-bedroom property in Hoi An.

They had their whole lives ahead of them. But their great adventure was tragically cut short when 33-year-old Greta, from Britain, and Arno, 36, from South Africa, were found dead, just three weeks after their engagement.

The couple's bodies were found in separate rooms of the Hoi An Silverbell Villa on Boxing Day.

Now, their cause of death has been confirmed as methanol poisoning. Police believe the source was a "homemade" bottle of limoncello bought from a local restaurant on Christmas Eve, according to The Sunday Times.

The tragedy comes five weeks after six people, including Melbourne teens and best friends Bianca Jones and Holly Bowles, died from drinking contaminated alcohol in Laos.

Watch: Warning to tourists as sixth traveller dies In Laos from methanol poisoning.

On Christmas Eve, Greta and Arno enjoyed dinner at a local restaurant and came home around 10pm. Two limoncello bottles were delivered to their hotel reception from a different venue.

The couple is believed to have drunk the alcohol and gone to bed.

When they woke on Christmas Day, Greta messaged her parents complaining of "the worst hangover ever" and said she was seeing "black spots" in her vision, per The Times.

Thinking it was just alcohol-related, they decided to try to sleep it off, unaware that they needed medical assistance. Tragically, staff found their bodies on Boxing Day morning.

According to media reports, Greta's body was discovered on her bed on the first floor while Arno was found in another room. The Guardian reported several empty alcohol bottles were found at the scene.

Police initially reported no signs of trauma in the villa and have since confirmed the cause of death was methanol poisoning.

Greta and Arno in their engagement announcement. Image: Facebook/Greta Otteson.

Greta's father Paul told The Times the couple were "so happy" together.

"We were so happy she picked Arno. He was such a lovely boy. Arno was a great musician, composer and lyricist," he said.

"They were a loving couple with their life ahead of them. The tributes we have had from around the world are unbelievable."

On social media, Greta and Arno appeared to be living their best lives, having moved to Hoi An last February.

In April, Greta said she was

They announced their engagement in December with a photo of them hugging at the beach.

"Life is kinda wild. I feel 12 but somehow I have a soon to be wife, living in a foreign country, rescued a dog from the meat market, developing a game, having a games night with friends and all while being 62% through life," Arno said in another post that month.

The restaurant that is believed to have delivered the limoncello is reportedly still open, and no arrests have been made. The couple's families want those responsible to be held accountable.

"The families remain fully confident in the ability of the Vietnamese authorities to conduct a fair and thorough investigation, and we ask for privacy as this process unfolds," Paul said.

View this post on Instagram

A post shared by Silverbell Hội An, Homestay (@hoian.silverbell)

What is methanol poisoning? And what does it do to you?

Methanol is extremely poisonous. Sometimes even as little as a shot can be fatal.

The alcohol is odourless, colourless and flammable, similar to ethanol, only it's not safe for drinking.

Dr Brad McKay told Mamamia's twice-daily news podcast, The Quicky earlier that methanol poisoning initially presents as similar to a hangover, making it difficult to pick up on.

"Often, if you've had a night out drinking, your body ends up metabolising all the alcohol first. Your liver takes on ethanol in preference… once that has been used up then your body starts to move on to metabolising methanol," he said.

This could be several hours after drinking, so the following day you could feel a lot worse.

Methanol is toxic because of how our body metabolises it.

While ethanol is metabolised into a toxic compound called acetaldehyde, it rapidly converts to acetate (which is found in common vinegar and is something the body can use for several purposes).

However, methanol breaks down into other compounds that are far more hazardous.

Methanol is metabolised into formaldehyde (a chemical used for embalming corpses) and formic acid (the chemical that gives ant bites their sting), Dr McKay said.

"You do only need that small amount to cause a problem. It's highly poisonous," he said.

Methanol poisoning can cause nausea, vomiting and abdominal pain. Blindness is common and often permanent too, as is organ damage.

Melbourne best friends Holly Bowles and Bianca Jones, both 19, died from methanol poisoning in Laos. Image: Facebook.

Symptoms of methanol poisoning.

In its early stages, methanol poisoning may just seem like you're drunk. Pay attention if your intoxication levels feel disproportionate to how much alcohol you have consumed.

Severe symptoms do not usually occur until 12 to 24 hours after consumption.

Methanol poisoning symptoms can include:

• abdominal pain

• nausea

• vomiting

• breathing difficulty

• vision problems (blindness, dilated pupils, flashes of light, tunnel vision, changes in colour perception, blurred vision)

• seizures

• and/or comas

Smartraveller urges Australians to be alert to drink origins and spiking, particularly with spirit-based drinks, including cocktails.

Contaminated drinks could include local home-brewed spirits, spirit-based drinks and brand-name alcohol, as labels on bottles aren't always accurate.

To protect yourself from drink poisoning, Smartraveller says:

• consider the risks when drinking alcoholic beverages

• be careful drinking cocktails and drinks made with spirits

• drink only at reputable licensed premises

• and avoid homemade alcoholic drinks.

Feature image: Facebook/Greta Otteson.