we have lived in two homes which backed on to Berowra Valley since 1973 - and never has there been a bush fire close by. That doesn't mean it won't happen one day - but we are fortunate that we have a VERY diligent local, Bush Fire Brigade in Hornsby Heights who are passionate about hazard reduction burning. If there is no undergrowth to burn, then out of control bush fires won't happen.
It does depend on where you're looking to live, for example some properties along Galston Rd & Somerville Rd are in a 'Flame Zone' which is obviously a highly rated fire prone area. However there are streets within the Hornsby Heights area like Bushlands Av or Evans Rd which have homes that are only in the 'Buffer Zone'.