New Zealand's ambassador to Thailand says violence has spread beyond the capital Bangkok to other tourist areas.
The Ministry of Foreign Affairs is now advising New Zealanders against non-essential travel anywhere in the country, due to increased civil unrest.
The ambassador, Bede Corry, says there's a high chance the safety of tourists could be compromised if they travelled to the country.
"We are seeing in northern cities popular with tourists spontaneous rallies, road blocks, disruption to civic services and, most worryingly of all, an ongoing pattern of grenade attacks."
Mr Corry says while foreigners are not being targeted in the attacks, tourists could easily get caught up in the violence unintentionally. The Ministry of Foreign Affairs and Trade is advising New Zealanders not to travel to any parts of Thailand due to increased civil unrest in the country.
The Ministry had been advising against non-essential travel to Bangkok, but on Wednesday night upgraded the security risk to high for the whole country.
It knows of about 1,000 New Zealanders in Thailand, but a spokesperson says the real figure is likely to be closer to 2,000.
Holidaymakers' plans disrupted
Thousands of New Zealanders have had their travel plans disrupted.
House of Travel retail director Brent Thomas says most holidaymakers are choosing to defer their holidays to Thailand, and those who are cancelling are re-booking to travel elsewhere.
Tourists already staying in the disrupted areas of Thailand have been moved to safer hotels.
Mr Thomas says most travellers have insurance, but those who do not have it are not eligible for refunds.
Soldier killed in Bangkok clash
In the latest incident, a soldier was killed as troops clashed with red-shirt protesters in Bangkok, apparently shot by mistake by one of his colleagues.
At least 10 others were injured on the capital's outskirts after troops fired warning shots to try to halt thousands of the protesters, who are demanding the government's resignation.
Two motorcycles carrying protesters was allowed through protesters lines, a photojournalist has told the BBC. An armed policeman then appears to have opened fire and shot the soldier in the head.
The security forces are armed with both rubber-coated bullets and live ammunition. Television pictures showed officers levelling their weapons in the direction of the protesters after earlier firing warning shots into the air.
The protesters currently occupy a swathe of Bangkok from the commercial district running south to the business district.
Prime Minister Abhisit Vejjajiva has so far rejected their demand for him to dissolve parliament.