The woman, Tracy Ann Walton, 51, had complained of feeling the heat and humidity on Phuket, said Patong policeman Colonel Thawatchai Srimai.
An emergency team from Patong Hospital was called to Patong Beach Resort about 2am on Tuesday. The Australian was found on the floor, next to her bed.
It is believes she suffered from high blood pressure and diabetes. Her body is at Patong Hospital.
Travelling overseas may have been a new experience for Ms Walton. Her passport was issued on August 12.
A second Australian, Tristan Sheridan, died within hours on Tuesday morning after his motorcycle crashed into a pole in Patong about 5.30am.
He was taken to Patong Hospital then transferred to Vachira Phuket Hospital in Phuket City, where he died at 8.52am.
Although the number of Australian visitors to Phuket has tripled since the 2004 tsunami, the Australian government has opted not to open a fully-staffed consulate.
Instead, a replacement is being sought for Australian honorary consul, Larry Cunningham, who retires on September 30.
Mr Cunningham said this week that as many as 50 Australians die on Phuket each year and up to 200 are arrested by police or Immigration officials.
More than 300,000 Australian holidaymakers come to Phuket each year. Several nations with smaller visitor numbers already have fully-staffed consulates on the island.
Other honorary consuls on Phuket are surprised that Australia does not have full-time facilities to assist Australian holidaymakers and expat residents.
Mr Cunningham was appointed after the tsunami when Australian officials realised that Phuket was too far from Bangkok for envoys based there to provide fast, efficient care.
However, the present Australian government appears to have taken no account of the vast surge in visitors from Australia to Phuket in the ensuing years.
Delivering assistance in tragedies becomes more difficult from Bangkok. Mr Cunningham's exceptional work rate has been acknowledged by the Australian Ambassador, James Wise.
Unless an equally skilled replacement for Mr Cunningham is found speedily, expat Australians on Phuket are likely to have to travel to Bangkok for passport renewals and other services.