Yes it can kill your tree. Usually the active ingredient in weed and feed is 2/4D which can be brutal to fruit trees. Depends on the concentration but you may get lucky in this time of year trees are about to lose their leaves in most locations anyway. Don't think there is much you can do now but wait until next year and see how it goes.

The main ingredients in the weed not feed I'm using are MCPA and Dicamba. It's also spring here in Australia. I didn't get much on the plants and immediately after I did I flushed them down with water.

Weed n feed*

I'm not sure about MCPA and Dicamba and how they effect fruit trees. I just know 2/4D is really tough on them. Spring is likely a worse time to have this happen due to the fact that leaves are or will be actively growing. If something happens in spring to stop growth the trees will have a hard time making through summer. At least in the fall the leaves are not growing and are soon to drop.

My concern would be uptake by the tree roots. I don't know how active those materials are thru the roots. But many herbicides are active in that manner.