The Northern Territory is not known for amazing climbing. A bigger population, cooler weather, better access, and climbing being just generally better accepted and more popular and there could be a lot of climbing in the Territory as there is plenty of rock about. But, as it stands, it is hot, even in the dry season, the population is relatively small and dispersed, and land management agencies Australia wide seem to frown on rock climbing.
In the Darwin area, the only place we found to "climb" was the Dripstone Cliffs at Cassuarina Coastal Park. These are way too short for anything but bouldering, and, were the rock more solid than children's crayons you could have some good bouldering sessions here as the rock is steep, alternately pocketed and juggy with great landings on a sandy beach. But, and it is a big but, the rock is incredibly soft and friable. A standard bouldering session results in dozens of broken holds.
About 120 kilometres south of Darwin down the Dorat Road is a little area called Robin Falls. There are some free campsites along the river side and a short track leads along the creek to a small two drop waterfall (Robin Falls). The tourists stagger up the semi-rough track to the falls, and, on either side of the valley there are a few sport climbs. We only did one climb here (School Teacher/Quartz Flake) so can't really comment on any other climbs. Anything on the left side of the valley (looking upstream) is baking in the sun, but the right side is shady. There are only nine climbs in total so I don't think the area is that popular.
A further 60 km south is Hayes Creek. There is a roadhouse and nice low-key caravan park ($15 night for an unpowered site) and you can walk to the climbing areas from the caravan park in under half an hour. The climbing is surprisingly good. Up the main valley, it is reminiscent of sandstone climbing in the Blue Mountains of NSW with sandstone pillars, cracks, and corners. Most climbs are gear climbs (no anchors) with walk-offs, although there are a few scattered routes with a couple of widely spaced fixed hangars. We only climbed on the right hand side of the gully, the on-line guidebook directions to "wade like drunken dinosaurs directly across the swamp to the crag" had something to do with us skipping the left hand side where there are only four established climbs.
There is a reasonably well beaten in trail if you walk to the swimming hole from the caravan park, cross the creek, head upstream for about 30 seconds and then walk directly up a small slope to a pile of rotting cans, bottles, and metal sheeting. The track then wanders up the valley and turns upslope just after a large boulder and brings you out right below The Nursery crag. The track is reasonable to The Nursery, The Sanctuary, and Sports Plus/Vodka Buttress, but beyond these crags, there is no track, and the grass is very scratchy and itchy (long pants and even long sleeved shirt required to prevent the "Hayes Creek itch"). We marked the access to The Apartments (reached before The Nursery) with a small cairn but, if anyone actually reads this and goes climbing there, the cairn may well be gone. You can actually see the orange wall of The Apartments from the valley bottom trail but there is no trail up to the base.
We climbed a variety of routes in the main four areas listed above and most were very good routes with reasonable to excellent protection. However, we did back off a couple of routes with rotten rock and/or poor to non-existent protection and there are numerous large and scary loose blocks laying randomly about on the top of many routes so, as with any climbing area in Australia, you can have a fun day out or a major epic. The problem with the climbing at Hayes Creek, which is strangely not mentioned in the guidebook at all, is that it is in the sun, almost all day. Even in mid-July, the acme of the dry season it was baking hot and almost impossible to climb in the middle of the day. The best strategy we found was to leave the campground around 1.00 pm to walk into the climbs. By the time we arrived, there were a few climbs in the shade and as the afternoon progressed more climbs got shady. It is dark by 7.00 pm so you really need to leave to walk back by about 6.30 pm. The track, such as it is, is a bit rough to walk out in the dark.
Downstream from the main valley is a small rocky valley which the owners of the caravan park call Butterfly Gap - there are hundreds and hundreds of butterflies - but climbers know as Spider Gully. Apparently this area was developed around 2006 by a couple of local climbers, one of whom was killed in a fall while doing route development at Robin Falls. Access is easy, follow the old road downstream, cross the river near a big sign (pointing to Butterfly Gap), walk a further five minutes downstream and look for a foot track heading up into the narrow rocky valley on your left. A couple of road markers have been stacked here.
Spider Gully is shady all day, but suffers from mosquitoes and humidity instead of sun and mosquitoes. The rock is very different to that found in the main valley. I'm not sure of the exact (even inexact) geologic origins, but, it is the kind of rock that fractures with sharp angles and is generally pretty steep. If you've ever climbed at some of the lower areas in Vantage (Washington), you'll find it very similar. Spider Gully is probably the prototypical Australian crag, which, if you had dared to develop a crag like this in North America any time in the last 20 years (certainly in 2006) you would have been vilified by the climbing community. The bolts are a mixture of carrots, occasional ring bolts, and Fixe hangars not all of which are appropriate for the type of rock. Anchors are inexplicably placed way back on loose ledges and are not amenable to lowering off. Cruxes off the ground are poorly protected, and carrot bolts are placed with no thought to having a clipping stance. Despite Spider Gully being a "sport climbing" area - usually a sure fire route to popularity - there didn't seem to be much climber traffic and the routes we climbed were pretty dirty. Make your own judgement.
Another 100 km or so further south and 22 km west of Pine Creek, Umbrawarra Gorge has some gear climbs scattered up both sides of the valley. The left hand side (looking downstream) gets sun early in the morning and bakes for the rest of the day. Consequently, we did no climbing on that side. The right hand side, however, has shade from about 11.00 am and there are a few good traditional climbs scattered along here. Some take reasonable protection, some offer long run-outs. There are no anchors, and the top of the cliffs is quite loose in some parts making anchor building challenging. A few routes happen to have handy trees on top which can be used for anchors. There is a basic camping area and a good day long excursion can be had by following the gorge all the way downstream to the end. To get completely to the end of the gorge and out onto the savannah you need to swim a long pool between narrow rock walls.
South of Pine Creek, we could find no references to any climbing, so, that's it until you get to the Alice Springs area.