A young Australian woman was the latest confirmed death, and her friend was fighting for her life, Prime Minister Anthony Albanese said.
Two Danish citizens and an American had also died, officials said, after what media said was a night out in Vang Vieng where they drank possibly tainted alcohol.
The group of about a dozen tourists became ill after going out on November 12, according to British and Australian media.
"Tragically, Bianca Jones has lost her life. Our first thoughts at this moment are with her family and friends who are grieving a terrible and cruel loss," Australian Prime Minister Anthony Albanese told parliament.
"We also take this moment to say that we are thinking of Bianca's friend Holly Bowles who is fighting for her life," he said, without giving further details.
Holly was on "life support" in a hospital in Bangkok, her father Shaun Bowles told Australia's Nine News on Wednesday.
At the Bangkok hospital where Bowles was reportedly receiving treatment, staff said they could not confirm her presence.
Denmark's foreign ministry said in a statement Thursday that two Danish citizens had died in Laos, without providing further information.
The Vietnamese manager of the Nana Backpackers Hostel in Vang Vieng where Australian media said the two Australian women were staying has been detained for questioning, the Laos tourist police told AFP.
No charges have been made, however, as police are still "investigating," an official said, speaking on condition of anonymity.
'Unimaginable time'
The US State Department also confirmed the "death of a U.S. citizen in Vang Vieng, Laos," and said it was "closely monitoring the situation and providing consular assistance."
It did not give details on the date or cause of death.
New Zealand's embassy in Bangkok said it had been contacted by one of its citizens "who was unwell and may be a victim of methanol poisoning in Laos".
The young Australian's bereaved family said in a statement to Australia's Herald Sun newspaper that they "are comforted by the knowledge that her incredible spirit touched so many lives during her time with us".
"The kindness shown to our family during this unimaginable time has been truly humbling."
Vang Vieng has been a fixture on the Southeast Asia backpacker trail since Laos' secretive communist rulers opened the country to tourism decades ago.
The town was once notorious for backpackers behaving badly at jungle parties and has since re-branded as an eco-tourism destination.
On their travel advice websites for Laos, UK and Australian authorities warn their citizens to beware of methanol poisoning while consuming alcohol in Laos.
Methanol can be added to liquor to increase its potency, but can cause blindness, liver damage and death.
In neighbouring Thailand, at least six people died and more than 20 were hospitalised after drinking methanol-laced bootleg alcohol in August.
Backpacker hostel
Bowles and Jones, both aged 19 from Melbourne, became unwell while staying at the Nana Backpackers Hostel in Vang Vieng last week, Australian media reported.
The women drank at the hostel's bar before they went out for the evening, the Sydney Morning Herald newspaper reported.
They failed to check out on November 13, when hostel staff rushed the pair to the hospital.
The Facebook and Instagram pages of the hostel had been deactivated as of Thursday and it was no longer taking bookings on websites.
AFP was unable to reach the hostel for comment.
UK media reported one British woman was also in hospital in Bangkok after drinking in Vang Vieng.
AFP has contacted the UK's embassies in Thailand and Laos for comment.
