Perth has a population of approximately 1,700,000. 1/3000 chance = 0.0003%. Let's assume for for the sake of this argument that 50% of the population have engaged in unprotected oral sex at some point in their lives. Therefore, according to this equation, there are approximately 283 people living with HIV transmitted via oral sex in Perth at this moment in time. Personally, I think that's 283 too many.
In some areas of the United States, oral sex has been shown to be responsible for as many as 15% of syphilis cases. The World Health Organisation states that cunnilingus is a "possibly unsafe" activity without the use of a barrier, whilst fellatio is considered an "unsafe" activity without the use of a barrier. The difference between the two definitions in terms of safety lies in the fact that pre-ejeaculate carries a much higher viral load than vaginal secretions, therefore it is easier (and more likely) to transmit a virus or infection via fellatio due to the viral load in pre-cum.
You can also transmit other STD's like herpes, syphilis, hepatitus B, molluscum contagiosum, and yeast infections during fellatio and cunnilingus. HIV is also risk. It is also theoretically possible (but almost unheard-of) to transmit gonorrhea or chlamydia during unprotected oral sex.
x