I was in the area on the weekend, walked down the Link track from Ku-ring-gai to the GNW by Calna Creek, along the GNW towards Berowra Waters, then back out up the Berowra Link Track. There's definitely some muddy and slippery areas and plenty of leeches about, but the creek crossings weren't affected by the rain.
For my money the better walk in than from the Gorge Rd would be down from Berowra (Crowley Rd) on the Link track, go part the track junction and stop to look at the view from Naa Badu Lookout, then descend down to the creek and along to Crosslands. You can put it together from
here and
here. It's be about 5 km from Berowra to Crosslands, and mostly downhill.