That will depend on which mel it is. There are so many and from what I can work out some tolerate pruning and root pruning better than others.
I generally cut up to 3/4 of the roots of all my trees, natives included.
I tend to go for broke and I'd cut above where they have knees (too long in the first tube before potting on

) but that won't leave much root and i can't guarantee it will survive.
If in doubt, cut a few of the roots short and leave the others long enough to sustain the tree while the shortened ones grow new feeder roots then cut the others the following year.