Australian classification laws are in the process of being overhauled, an overhaul which may see their powers increased. But few people know exactly what is currently banned in Australia.

As it turns out, much adult content (one of the most popular subjects on the internet) is banned or bannable. There seems a real gap between what is notionally prohibited and what Australians regularly consume.

This picture is made all the more baffling by the secretive nature of Australia's obscenity guidelines.

Classification laws are among the most powerful in the land and underpin Customs importation laws and Senator Conroy's proposed mandatory internet filter which will start to be implemented next month.

Discussion of what might or might not be excluded by an internet filter hits a road block when it comes to "Refused Classification" or "RC" - a term that is at best vague. We are told that the promised filter will be set to catch RC content. However, few people (including some of the legislators) actually know what RC means.

"Refused Classification" is also the basis for the term "illegal pornography", a description now appearing on Australian airport landing cards. All travellers must declare potentially-RC items when arriving in Australia. However, because neither the Classification Board nor Customs publish their guidelines, no-one knows what might be potentially-RC - until they are told that they have broken the rules. The penalties for breaching these invisible regulations include fines, deportation and imprisonment.

The official classification Code initially echoes the Australian Constitution by opening with the phrase "adults should be able to read, hear and see what they want". However, the succeeding 21 pages are spent qualifying this statement. You can read it in full, here.

It is generally believed that the Classification Board decides upon what's acceptable using common sense and experience. Board members are, according to the Code, the definers of "the standards of morality, decency and propriety generally accepted by reasonable adults". However, the board's internal Guidelines are very specific and the Board members' main job is to match submitted content with what appears in its Guidelines. With every borderline decision, the Guidelines have become more prescriptive.

The Board briefs Customs on these Guidelines and periodically gives scaled-down presentations to adult publishers in order to clarify the broad, grey, contradictory area that the Code defines. But other than that, no-one knows what they are in detail... until they get caught flouting them.

One adult publisher has sought to document the official Guidelines, based on briefings by the Board and noting over time the reasons given for submitted content being Refused Classification. According to a former Classification Board member, this document accurately reflects the official but unpublished Guidelines.

According to the document, material including any of the following is refused classification by the Classification Board in Australia. Such publications are also deemed "objectionable goods" under subregulation 4A(1A) of the Customs (Prohibited Imports) Regulations 1956 (the Customs Import Regulations).

(Italics below indicate direct quotes from the adult industry document.)

No depiction of violence, sexual violence, sexualised violence or coercion is allowed in the category. It does not allow sexually assaultive language. Nor does it allow consensual depictions which purposefully demean anyone involved in that activity for the enjoyment of viewers.

Fetishes such as body piercing [and tattooing], application of substances such as candle wax, 'golden showers', bondage, spanking or fisting are not permitted. As the category is restricted to activity between consenting adults, it does not permit any depictions of non-adult persons, including those aged 16 or 17, nor of adult persons who look like they are under 18 years.

While a ban on the sexual depiction of minors will have strong community support, there's a much greyer area involving adults or even animated characters who look young. Most adult movies (online or DVD) come from America and carry official government statements guaranteeing that all participants are over 18. These cut no ice in Australia. Furthermore, Hentai Manga (Japanese sexual comics) are so popular in Japan that they are freely available for browsing in 7-11 convenience stores and read openly on trains. But they are RC in Australia - potentially a rude shock for Japanese tourists visiting with such comics in their luggage.

Note too, that over the past year, the Classification Board has started using breast size as a criterion in defining child pornography: a less than precise indicator.

Beyond questions of age and appearance, some of the guidelines are what you might expect: Depictions of bestiality, necrophilia, incest, drug use, paedophilia, detailed instruction or promotion in matters of crime, high-impact violence and cruelty are all grounds for an RC rating.

However, things get less obvious when it comes to violence associated with sex:

Violence: rough or injurious physical force, action, or treatment. This includes actual violence (shooting, punching, pushing, throwing a person, etc), implied violence (gunshot sound effect, news article, mugshots), aftermath of violence (person with injury, dead body), threat of violence ("I'll kill you"), and violent behavior (woman holding gun while engaged in sex with man). Note down ANY and ALL violence, even if it looks contrived or unrealistic (plastic swords, etc). Depictions of dead people are also not permitted.

The implied violence comment is so strict that it renders virtually all crossover drama/porn films (those that ape cop shows, fantasy films and drama, but with added full sex scenes).

Adult videos have for instance been Refused Classification for showing a gun on a table or for showing a headline in a newspaper describing a murder. One video was refused classification because it was about people looking for a friend that had been kidnapped - even though the kidnapping was never shown. Another was rated RC because a character simply had a black eye. Another was rejected because of a scene showing a doctor putting on a pair of rubber gloves.

Sexual Violence: Spanking, choking, pinching, stepping on the face, hair pulling (either as a violent act or consensual fetish act), rough or 'man' handling, face slapping, and general rough play are all prohibited;

General rough play is a description that could be attributed to virtually every film that features sex. Only one spank (as in a slap on the bum) is allowed at any one time. And that can't be very hard. All BDSM is banned.

Sexualised Violence: being sexually aroused by violence or using violence with the intent to arouse; is also out of bounds, with an apparently very literal interpretation being adopted.

Language is another tricky area.

Sexually assaultive language - a tone of voice or language that is demeaning and disparaging. Eg. Calling someone a whore or slut, or telling them to do something demeaning in a disparaging way. This does not include 'dirty talk'.

Depiction of less-conventional sexual behaviour is also proscribed: Golden Showers: Urinating on one self or another person is objectionable. Female ejaculation or 'squirting' is considered to be golden showers. The latter is controversial with medical discussion suggesting female ejaculation is a minority but normal sexual response.

Beyond questions of what is and isn't refused classification there's the issue of enforcement.

With over 20 million visitors coming through Australian airports every year, and with many carrying smartphones and laptops, the chances of people breaking Australian law without realising it are very high. However, the amount of content involved renders searching for violators practically unpoliceable. Customs told us,"Customs and Border Protection made 1,373 detections of objectionable material in the 2009-10 financial year, across all cargo and passenger streams. Of these detections, records indicate 50 per cent were made in the passenger stream.

"54 cases were prosecuted, including 47 cases involving child pornography. Penalties ranged from $200 to $20,000. Sentences included good behaviour bonds ranging between five months and two years, to imprisonment ranging from six days (time served) and three years and nine months."

These figures are miniscule when compared with quarantine and other-contraband smuggling infractions. It does not appear as though Customs is too fussed about searching for "illegal pornography".

Travellers do not know the Australian definition of objectionable pornography and are likely to believe that world famous 'porn stars' such as Belladonna, Rocco Siffredi, Cytheria, Guage, Angel Long are legal, even though each is renowned for practices that are considered "offensive" in Australia.

Nevertheless Porn is still one of the most searched for and consumed content areas of the internet. And if everything mentioned above is considered RC it would subsequently need to be placed on Australia's internet filter blacklist and censored. However, there are reckoned to be some 600,000,000 adult-oriented, sex-related pages on the internet (with many new ones appearing daily) so investigating and blacklisting them all would seem impractical at best.

Given the secrecy around the classification detail, it's hard to judge whether this is an approach truly in line with community standards. Is this what the Australian public wants? And is it a standard Australians want applied to an internet filter?

Nick Ross is the ABC's technology and games editor.