Eucalyptus is technically poisonous, but it depends on amount.
Concentrated Eucalyptus Oil, like an essential oil, should always be kept out of reach of children and have a childproof cap, for example (http://www.poisons.co.nz/fact.php?f=27). It is a dangerous and confusing point because, like clove oil which is also poisonous in sufficient amounts, it is used in various remedies in small amounts. Of course, when you have a concentrated oil, "sufficient amounts" is relatively small. Putting a leaf in some tea is quite different.
There are a number of places on the web selling Eucalyptus tea and giving recipes, but the general recommendation is not to drink too much or make it too strong. The general recommended amount is 1/2 tsp dried leaves or one torn up fresh leaf per cup of water, and 1-3 cups per day.
This responsible looking website notes some safety issues: http://www.eucalyptusoil.com/safety
This website gives more details of the chemicals found in Eucalyptus, and tells you how to make a tea. It also gives a list of medical conditions which indicate you shouldn't drink Eucalyptus tea. http://www.livestrong.com/article/523149-what-is-eucalyptus-tea-good-for/
Another website also contraindicates eucalyptus for use with some drugs - 5-Fluorouracil, Pentobarbital, Amphetamine (I can't provide source because not enough reputation to post the extra link)
Note: Many things that are contained in various herbal remedies and so on contain active ingredients that can ... have all sorts of effects. So everyone should research them themselves or check with a Natural Health Practitioner or doctor first before taking them with medications, or giving them to small children, pregnant women, asthmatics or any high-risk health group.