Probably just sit back and enjoy it!
To clarify the situation for those unfamiliar with mistletoes, these are parasitic plants which attach themselves to other host plants (usually onto the branches, but root parasites are known) and extract most of their growth needs (water, nutrients) from the hosts. They can occur on almost any plant but are most commonly seen on eucalypts as ovoid-shaped clumps of foliage hanging from the tree's branches.
Many mistletoes are host specific and their foliage may mimic that of the host tree.
Mistletoes and their hosts have evolved together and, unless the host tree is unhealthy, one or two (or more) mistletoe clumps should not cause any problems. In fact they can be enjoyed as part of the overall garden environment. They often have attractive fruit and flowers and, if you're lucky, they will attract the beautiful mistletoe bird which lives almost exclusively on the fruit.
|Left: A clump of mistletoe on a Eucalyptus sp. Right: The magnificent Western Australian Christmas Tree (Nuytsia floribunda) is a root parasite.
Photos: Brian Walters and Owen Roberts.
If mistletoes appear to be causing unacceptable stress on a tree, then it is usually a job for a professional tree surgeon.