Hi Clare,
Nillumbik is a lovely area to live. It's quite diverse, if you have a look at the Shire of Nillumbik's website
Nillumbik Shire Council Home Page there's some good information on suburb profiles. The more northern areas like Panton Hill, Cottles Bridge and St Andrews (and so on) are more rural and have more farm/horse properties, whereas if you come back towards Diamond Creek, Eltham, Greensborough you get a mix of trees/parkland and smaller residential blocks. Eltham and Warrandyte (Nillumbik touches on North Warrandyte) are well known for having lots of creative residents which make them great suburbs to live in, or nearby, if you enjoy visiting galleries, markets, festivals etc. There's good access to the Western Ring Road from Nillumbik which makes it handy to commute to the airport/northern suburbs too. I hope this helps! Pls post back if there's any more specific info you need. Best wishes, Mel :)