I would reiterate Snowzone’s advice – don’t rely on water being available at present.
There is a soak/spring (GDA94 616006 5869536) about 600m south (and 150m lower) along the track from the main cave camp area. However, this is NOT reliable after extended dry.
From the cave, the creek directly east is difficult to access (about 300m below) through thick scrub and reasonably steep terrain. I would not rely on it for water.

- Fortress camping cave - May 2008
There is no water anywhere near Mt Thackery campsite.
If you
really don’t want to carry water, consider doing a water drop at GDA94 616453 5868516 on Victoria Range Rd where the Fortress Track emerges (about 2km on track from the cave). You could also drop at the Mt Thackery campsite (since you'd be passing it in the vehicle to do the other drop).