The coup leaders dismissed Prime Minister Thaksin Shinawatra, who is in New York for a United Nations meeting, and revoked the constitution, promising a swift return to democracy after political reforms.
Tanks and troops took over Government House in the nation's first coup in 15 years. A coup spokesman said the army and police were in control of the capital and surrounding provinces.
But life in most of Bangkok continued much as usual with traffic moving through the streets and the airport operating normally.
Australia’s Department of Foreign Affairs and Trade (DFAT) advised its citizens to reconsider travelling to Thailand and issued a warning for those in Bangkok:
“We strongly advise Australians to reconsider their need to travel to Thailand until the situation is clarified. Australians in Thailand should exercise extreme caution. You should avoid concentrations of military personnel or military activity,” DFAT said on its website.
The department renewed its travel advice for Thailand earlier this week, urging visitors to exercise caution in southern Thailand after a bombing in the resort town of Hat Yai four days ago killed five people.
“We strongly advise you to reconsider your need to travel at this time to the southern provinces of Yala, Pattani, Narathiwat and Songkhla or overland to and from the Malaysian border through these provinces, due to high levels of ongoing violence in these regions, including terrorist attacks,” a posting on the DFAT website said.
Staff at the Australian embassy in Bangkok had been told not to send their children to school until the situation was clarified.
Meanwhile, chairman of the Australia-Thailand Institute and former deputy prime minister, Tim Fisher, says all Australians should take the travel advisory very seriously.
"(It) has all the hallmarks of a back-to-the-future coup d'etat," Mr Fisher said.
"I urge all people to follow closely the updated DFAT travel advisories which have been issued already, and will continue to be updated as further information comes to hand."
Australian tourist Steve Williams, from Melbourne, said he was due to leave Thailand at the end of next week but decided to leave earlier when he heard news of the coup from his hotel room.
"We decided to take matters into our own hands after ringing the Australian Embassy, and get to the airport and try to get out as quickly as we could," he told Southern Cross Radio.
"On our way to the airport, the traffic was very, very light and there were lots of military personnel trucks and tanks on the streets of Bangkok."
Mr Williams said he would leave Thailand even if the situation was resolved and Thai life returned to normal.
"We're pretty well hell-bent on coming home because they did have some bombings earlier in the week, down south where we're going to Phuket," he said.
"We feel safer coming home. We've had a great two-and-a-half months, so we're happy with our trip so far."
Australia's Ambassador to Thailand, Bill Paterson, was on duty overnight with senior staff at the Australian Embassy in central Bangkok, monitoring moves by army officers against the government of Prime Minister Thaksin Shinawatra.
"We are regarding this as an emergency situation," an embassy spokesman said.