A new report by the National AIDS Commission said the number of HIV infections on the island had jumped almost 19 per cent to 3,778 this year, with another 597 cases.
"The figure is only the tip of the iceberg... we estimate the actual number of cases to hover around 7,000 as many cases were unreported," said the commission's spokesman in Bali, Nyoman Mangku Karmaya.
"Many people are reluctant to go for medical check-ups as there's still a stigma and discrimination against people living with HIV/AIDS," he told AFP.
The number of cases this year has risen sharply from 1,253 cases in 2006 and 590 in 2004.
Karmaya said about 25 per cent of the estimated 8,800 sex workers in Bali were infected with HIV this year, up from 23 per cent last year, adding that the virus was transmitted from clients who refused to use condoms.
While describing the trend as "worrying", he said most tourists visit Bali for the culture and scenery, not for sex.
"There might be some who get acquainted with the sex workers, but they're aware about using condoms," Karmaya said.
However, he warned that there could be more than 840 new infections in Bali every year if the spread is not contained."We will increase our efforts to provide more information and educate the sex workers as well as the public. We'll also intensify campaigns promoting condom use," he added.More than a million tourists visit Bali every year to enjoy its surf beaches, nightlife and Hindu culture.