Far too hard to answer this one on a web site!
First make the decision what sort of harvesting technique you are going to employ, and then prune the trees to suit.
Hand picked trees need to be reachable by hand, preferably from the ground as ladders are always a bit of a problem. 3m in height should be big enough. There may be no need to leave much of a clear trunk, but this tends to depend on the variety and its growth pattern.
If you are going to go for machine harvesting, it tends to depend on the type of machine. There are 4 main sorts around these days.
Hand held limb/leaf shakers and combs - held by an operator on the ground and used to knock the olives off the tree onto nets on the ground. Basically an extension of hand harvesting, so small trees work best. Open vase or vase-bush is the most common shape.
Trunk shakers - quite large machines that grip the trunk of a tree and give it a short, sharp shake to dislodge the fruit so if falls either onto the ground in into nets, or into a catching umbrella. These are known to cause some root and trunk damage, and usually require an open vase shape and a clear trunk to 1.2m.
Straddle harvesters - modified grape harvesters that straddle the row and drive along over the trees, knocking the fruit off with a series of beating rods inside the centre of the machine. This technique means treating the olive trees like a hedge, so pruning needs to encourage the limbs to go where the harvester wants them. The machine will tend to rip off limbs it doesn't like. Smaller, single side straddle harvesters are being developed, but the pruning remains the same. In some machines the fruit is caught on a conveyor inside the machine and automatically loaded into crates.
Hawke Harvester - a new innovation from Wauchope in NSW. This machine has a large, circular catching frame that fits under the tree, and a series of vibrating rods that work up from the bottom of the tree to the top, combing the fruit off. Vase or single leader seems to work okay, but the technology is still evolving.
So, to answer the question - how to prune. If you have never done it, please call in somebody with experience to give you a few pointers before attacking your tree. Pruning an olive tree is a process, not a one-off project. The tree needs to be shaped and renewed regularly. You may decide to prune annually, or perhaps every two, three or four years. There are lots of theories and philosophies in the correct way to prune and olive tree, so expect to come across a few contradictory ideas. Trees need to be trained, and if neglected will soon get out of shape and need serious pruning to get them back into bearing.
Here is some background information prepared by Matt Starczak to give you a start: www.geocities.com/starczaks/pruning/pruning.html
The CSIRO publish a book called Pruning and Training Systems for Modern Olive Growing by Riccardo Gucci & Claudio Cantini which is an Italian manual translated into English. This is well worth reading (and owning!) but bear in mind it is written about Italy, not Australia.
Please bear in mind that the strong ultra-violet light in Australia may mean that a different strategy may be required to here, with different techniques to pruning than those practiced in Italy or Spain, and that the high humidity in the Hunter Valley also causes problems which can be addressed by correct pruning.