If you're talking about the Blue Gum forest just below Katoomba, Flatfoot, then I grew up in the next set of ranges (Central West, 900-950m ASL.)
July is the Brass Monkey Weather. It starts to moderate in August, but not very much. September sees a sudden change in weather with the arrival of spring temperatures and the lack of frosts.
The temperature at 300m ASL would be different to that 900m up. If it's snowing 900m up, it'll be sleet or cold rain when it gets to you. The town I grew up in has hills on all four sides, and was very cold in winter as the cold air rolled down and settled in the valley every night, a bit like Lithgow, but there's a spot for the cold air to flow out of the Lithgow valley. In sheltered spots, it was warmer on top of the hills.
The overnight temperature would depend on wind and cloud cover more than most, if it's a still, clear night, you'd be very cold, but with a bit of wind and cloud cover, it could be mild IF there's somewhere for the cold air to flow to. I always plan on cold air settling into a valley overnight.
As John said, pack for below freezing -> better to be overly warm than the other option.
disclaimer: Have not been through the Blue Gum forest, so general location information only based upon 10 years of very cold winters and local(ish) experience.
Cheers,
eddie
edit: long term weather averages for Katoomba.
http://www.bom.gov.au/climate/averages/tables/cw_063039.shtmlCheck the temperature difference between Oberon/Jenolan State Forest and Jenolan Caves - the caves are in a steep valley, and average zero temps all through July and August, while Oberon/Forest average about 2°.