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Hey @Pakster!
Best time would be late winter (now) through to early Spring. Depending on where in Aus/NZ you are (and how frosty it is, such as Melbourne at the moment), hold off until the late Spring just so you don't stress the tree out too much. 
Hi @Pakster,
It's great to see that @Remarka6le has already provided an answer to your question. I thought I'd add that it's best to avoid over-pruning. Don't remove too much of the canopy, as it will reduce the tree's growth and this year's crop. You should be looking to remove only unhealthy, unwanted and poorly positioned branches. Don't remove healthy foliage unless it is entirely out of shape. It's best to do a yearly prune instead of a hard prune which risks losing that year's crop.
As mentioned, the best time to prune is after any frosts have finished and before bud break. I'm sure you'll find this step-by-step guide useful: How to prune citrus trees.
Please let us know if you have further questions.
Mitchell
