Because we live in an area where the particular Blue Gum variety of Eucalyptus is plentiful, that is all I use.  I have used other varieties of flowering Eucalyptus, but none are as aromatic and lovely smelling as the Blue Gum.  The birds know the difference well.  I have always found it interesting that the Blue Gum trees start to bloom in late December, precisely the same time of year as the Galahs go to nest.  I make several trips a week to cut Eucalyptus for them, and I try for a generous portion with booms if I can.  When I put the leafy boughs with blooms in the flights, the Galahs are there instantly, chewing away on them, looking quite intoxicated.  Interesting to watch them take one leaf at a time, carefully carry it over and place it into the nest hole.  I keep bringing the Eucalpytus in for the duration of the breeding season, because they have such an interest in it, especially if they are going to produce a second clutch.  Before they lay a second clutch of eggs they will rebuild the nest a bit by adding fresh leaves. 

I am always diligent about attaching the Eucalyptus branches up fairly high in the flights, and as close as I can place it to the nest box.  Having the branch close to the nest box means they don’t have to haul it so far, as sometimes they drop the leaves on their way to the box.  If any of it falls on the ground, the Galahs will rarely haul it up from the ground to put in their nest.  Once the leaves become dry, I remove them if the branch is intact, because the birds are only interested in fresh green Eucalyptus.  For those that do not have access to fresh Eucalyptus leaves, most any non-toxic green leaf material can be utilized.