Basically the closer to the beach you want to be, the more it will cost you, and the more competition there will be for property, along with the inevitable blood bath and rent bidding wars.
We originally began to look along the Hornsby train line, Turramurra, Pymble, Wharringa etc. We saw a lot of dumps and some nice properties (usually owner occupied) in the region of $750pw, but you need to consider where you will be working and how easy it is to commute to CBD. If you are a little way from the train station, you probably need to drive and parking is not always good. The thing I found was the area seems to be devoid of footpaths so walking to the station would be restrictive.
Another worry was feeling isolated in these areas, you really do have to drive to go for a night out etc and if your partner does not work they could feel very isolated if you don't have a car to get out and about.
We ended up in Balmain, which suits us, loads of transport into the CBD as hubby works in CBD. Also great for a night out or to go back into the city to enjoy the theatres, cinema etc and a taxi ride is only about $20 dollars rather than $40+ for the Nothern Shores.
Don't discount the southern shore areas as there are some nice suburbs, and you will never be that far from a beach if you want to visit.
To be honest it is really very hard to recommend areas as what suits one just isn't right for another.
Have a look at this site (http://www.cityhobo.com/cities/sydney) gives a bit of an overview of different suburbs.