The regulations state in the section on "Burials" that:
A person who buries a body contained in a coffin must place the coffin so that its upper surface is not less than 900 millimetres below the natural surface level of the soil where it is buried (Clause 20). The basic intent here is that burial should be at such a depth as to avoid remains being easily disturbed either by people or by animals. There is also the need to contain the odours of decomposition. An exception may be approved following discussion with the local PHU. In the past some cemetery authorities have come up with innovative solutions that have been approved by the local PHU to ensure a grave is sealed and not accessible when the situation has prevented the minimal soil depth requirement from being met.Reference: The Public Health (Disposal of Bodies) Regulation 2002, page 13, section 5.
Basically the top of a coffin has the be at least 900mm (or 2.9 feet) below the surface, this allows for variations in the coffin size and disturbances or erosion of the top soil. How deep the total grave is depends on what the mourners and/or deceased want and the conditions.
There are three grave depths in the funeral industry. The first is 'tripple depth', this is the deepest of the grave depths. As the name suggests it can accomodate a total of three adult coffins (each on top of the other). Because of the depth this is the most expensive to dig, yet in some ways the most practical as other family can be buried in the same spot.
The next is 'double depth', which is the middle one. It can accomodate two coffins on top of each other and is not as expensive to dig. This grave type is about 2.1 meters in depth (or 6.8 feet). This is the grave depth most people think of. It is the one we see in most movies and TV shows and the closest to 6 feet.
The final depth is 'single depth'. This is the shallowest and only holds on coffin. It is also the cheapest and easiest to dig. To meet regulations the average single depth grave is about 3-4 feet deep.
Already we see that two out of the three grave depths are deeper than 6 feet (or 1.8 meters). Tripple depth is significantly deeper and double depth is noticeably deeper, being almost 7 feet. The final one, single depth, is actually shallower than 6 feet, being only about 3-4 feet deep.
So next time you hear about a grave being 6 feet deep or see one on TV you will know that this is untrue. Like many assumptions people make about the funeral industry it holds no reality or practicality. But it does sound nice, "six feet deep" has a better ring than "6.8 feet" or "900 millimeters".
It should be noted that this post is looking at NSW Health laws and regulations. This might not be applicable or even comparable to other Australian states let alone other countries.