Digging post holes in a speedy and precise way is a really common earthmoving task in residential, civil and commercial construction. This means there is literally an infinite number of different ways post hole digging can be done, and a heap of plant hire companies offering you a range of very low cost, to higher cost ways to dig a post hole. You can also buy one man earthdrill online.
The largest factor will in choosing the right machine and attachment for the job will be the type of rock or materials you are trying to dig through.
Harder materials, like rock, clay or limestone, will mean that you will most likely need to use an auger attachment affixed to something with a bit of grunt (a large engine) and a bit of purchase (tracks). A skid steer loader, large dingo digger or mini excavator would do the trick.
Basically, if you are going to dig a six in a hole in very firm material, you’re going to need the motor of an earthmoving machine to drive the auger through it. Conversely, if you are displacing really loose and light material like soil or sand, then you can get really wide holes by using just a hand-operated post-hole digger.
Even though auger attachments affixed to earthmoving machines are much more expensive to hire on a daily dry hire basis (compared to a hand-operated device). A skilled operator can whip around a site in a matter of minutes and puncture a large number of post holes. Hand-operated devices are much slower, so you more often see them on the DIY residential jobs, but they are perfect for a weekend warrior guy or gal.