AusBonsai

I recently picked one of these up from a nursery.

I'm happy with the trunk, but none of the branches.

How tolerant are these of A hard prune?

Also I'm in Melbourne and its the middle of summer. When is the best time to go hard on it?

These guys back bud proficiently and grow like crazy in general so hard pruning is not going to be an issue but you might want to hang off until winter to go really hard, after they drop autumn foliage I reckon - summer isn't really ideal. The good news is after that it will be grow back in no time next spring. Enjoy!

My 2c is that I'd be very very surprised if you didn't receive new buds and growth before the season's end.
I think there is enough time and personally I'd give it the chop now (if that's what you believe must be done).

Once Zelkova's go (they are late to bloom in spring) they are pretty aggressive tree's.

Anyone else have some thoughts on this?

Cut back my zelkova 2 weeks ago, and within days buds popped everywhere and we are having a bit of a hot spell this week, keeping it in the shade to protect new shoots. I choose to cut back if I can tell the tree is healthy and in a period of growth.

They respond very well to defoliation and now is a good time. if you cut off the branches you have enough time for them to grow and harden before winter sets in.

Here is one of mine that I defoliated recently.

viewtopic.php?f=114&t=13280&hilit=+zelk ... 50#p194950

As stated in this post I reckon I could probably do again but might leave it and focus on the ground layer.