Portions of the following information were taken from DiveDoc.net

The sinkhole at Allendale East is a unique feature in the sense that the main Mount Gambier-Port MacDonnell "Bay Road" splits centrally and diverts around both its eastern and western sides in the heart of this small township. After years of ongoing subsidences, the sinkhole seems to have stabilized as a 10 metre diameter collapse with a steep climb-down section on its southern side, leading to a crescent-shaped pool of crystal-clear water some 6 metres below ground level. The underwater region of interest to recreational cave divers basically consists of a single passage some 35 metres in length, dropping to a maximum depth of around 28 metres to the north-west of the entrance lake (probably under the road in that area). There is also a small dry crawl-down passage under the north-eastern wall.

Although the water is very clear, it is quite dark because it is shaded from direct sunlight, and very little aquatic life is to be found; in fact, the only life-forms recorded to date are some syncarids and yabbies (including at least one pure white one). Water samples collected by the author in 1981 indicated that the level of nitrates (as NO3) was around a moderate 17.1mg/l, and the cave was mapped in detail by Stan Bugg and Brian Cornell of SAUSS (Inc) in the early 1990s.

Allendale Sinkhole has been known since before the beginning of the 20th Century, when it was apparently used as a major watering hole for bullock wagons. The 3 August 1971 edition of the "Border Watch" newspaper reported that the cave "... was supposed to have appeared one night after a bullock wagon left one of the hotels in Allendale on its way to Port MacDonnell. The wagon, with its team and teamster, disappeared as the hole appeared, but the whole outfit was supposed to have "surfaced" again at Eight Mile Creek". Mention of the bullock team and dray was also made in Father Julian Tenison-Woods' "Biological Observations" of 1862.

An interesting early description was provided by the late Mr. W.A. Habner, of Port MacDonnell District Council, who reported that he had seen a basic map of the inside of the cave ... (it was like) "the cavity of a double tooth, with the main hole and two cavities", with one extending back towards the Allendale Hall (presumably today's popular diving passage) and the other heading in a south-easterly direction. He also said that the filling attempts first involved some 550 cubic yards of the biggest basalt rocks the Highways Department could handle, before many truckloads of loose paddock-stones followed. They only admitted defeat when the cave opened up again after NINE THOUSAND CUBIC YARDS of rock had been dumped into its entrance!

Unfortunately, no records of any pre-fill scuba dives have been located to date, and it seems unlikely that anyone dived it when one considers that the first recorded dives in the other sinkholes of the region took place during the very late 1950s.