Hey Andras,
I fish Narrabeen lakes plenty with my younger brother and one location that is guaranteed a catch is near the caravan park on the north-east side of the lake. He has a video on his youtube channel in this area, the best way to go about fishing here is to have the smallest hook you can with a small ball of bread on the tip, there are a few large fish about it can be hard to get them to bite before the little ones get to it first.
There is another location that often has good results this is near the Woolworths which has a higher chance of getting a bigger fish he also has made a video in this area but we only caught a few small tarwhine and bream. But I did have something large come off, it almost broke my rod.
The final location I can recommend is on the tip of the point off Wimbledon Avenue in Narrabeen, this part of the lake has similar small sized fish but we once hooked on a large jewfish that sadly broke off just before we landed it.
Good luck fishing!