Flood waters burst the banks of the Latrobe River and the open cut was flooded.

Jean Hattam wrote: the big floods made us so isolated – I can remember the milk coming to Yallourn by railway from Yarragon

Edgar Scott (Principal of Yallourn Tech College from 50s-70s) - sent in by his daughter, Jenny - "A natural disaster of great practical and economic impact, and of historical interest, occurred at Yallourn in December 1934. In brief, the event was the flooding of the Open Cut, and its being put out of production thereby for one year. In more detail the sequence of events was as follows:
Commencing at 7.30am on Thursday 2 Dec 1934, from leaden clouds appearing as just several feet above the chimney tops, heavy rain fell and continued until later than mid-night on Friday. The character of the rain was unusual; not violent noisy, bouncy, big drops, as often occur during thunderstorms; but much more like the shower in a modern bathroom, closely pitched continuously flowing fine jets, the audible result being a muffled unceasing roar, and the practical result being the deposition of 1100 points – now 275 mm – in and nearby Yallourn. What was more important was that more than three times that amount fell in the catchment area of the Yarra and Gippsland Rivers systems.
At about 11am on Friday morning, the road through Morwell Bridge was car running-board depth in water for about 300 yards. Not much later the road was closed to all traffic and remained so for two days, and the Morwell Bridge school located about 100 yards north of the Princes Highway and 100 yards west of the Morwell River, was lifted bodily from its stumps, floated downstream several hundred yards, and eventually its downstream journey was checked when it became wedged between large gum trees. Needless to say, the highway, and almost all other roads were impassible for varying periods, and it was years before the damage was completely repaired.
The siren at Yallourn Power Station had, for years, been used to signal shift change times, starting and cease work times, for the Yallourn area; and following a defined code of short and long blasts, to signal fires in various locations and "other " emergencies. It was no surprise to any within earshot that, at almost exactly 11 pm on Friday night, 3 Dec, the siren sounded the "series of long blasts" assigned to "any other emergency". It was evident that the emergency pertained to water in very large quantities, though it was not until daylight that the nature of the emergency became clear. The Open Cut was protected on the Latrobe & Morwell Rivers sides by levee banks, and that adjacent to the Latrobe was breached in a minor fashion early in the evening. As the flood level rose to a final height of 37 feet above normal, the levee was completely breached, and the siren was signal to the need for rescue of personnel. All were rescued, though several located on the Overburden Dump east of the Open Cut were isolated there for a day or so. The final result viewed in bright sunshine on Saturday morning showed the Open Cut full and level steady, the town water supply pontoon and pumps on the bend of the Latrobe nearest to Reservoir Hill completely wrecked and mostly somewhere downstream; the swimming pool somewhat downstream from the pumping station had disappeared; the weir, still further downstream, the flow-over type, was completely wrecked, mainly because of the battering received from floating trees; and the low-level bridge to Yallourn North was just not there anymore. With appropriate variations this story could be applied to most of Victoria east of Melbourne and south of the "Divide" .
One aspect of interest concerning the Open Cut was that inflow ceased at about 7.30am on Saturday morning, and by 7.30am Sunday morning, the top 17 feet had flowed out again! However there remained about 200 feet more, some 170feet of it being below river level, and the removal of this to a stage of the Cut being again operable occupied exactly one year. Not only was the Cut full of water, but every item of coal winning equipment was under the water!
There was much concern at the prospect of no coal, and predictably there were recriminations, "they" being blamed for failure to provide protective measures. Being regarded by some as an Act of God did not result in any heavenly statement of agreement or denial! Very soon causes and blame were replaced by measures of restoration. For more than a year the area was invaded by huge numbers of horses, drays, wagons scoops, ploughs, rollers and men. These came from far and near to re-establish roads, clear debris, open and put into service the disused Yallourn North Open Cut (which has continued to function to the present day, principally as coal supplier to A.P.M.) and to repair/re-erect damaged and destroyed structures. Much of this work was of the nature then carried out by farmers, and the opportunity to hire themselves and their equipment to the SEC was much appreciated by this group which had experienced some 5 years of depression.
The major task was that of Cut "de-watering", and the salvaging and re-conditioning of the equipment, including dredgers, locos, signalling and communication equipment. All the equipment was of electrical type, - it can be appreciated what a job it was to restore it after total immersion in muddy water for from 6 to 12 months. There were spares of much of the electrical equipment on hand, so replacement of some of the "electrics" was possible very soon after the de-watering was accomplished. The de-watering was done by obtaining from the Adelaide harbour Authority two very large "sand" pumps (sand or sludge pumps are capable of pumping water with a high solids content) which had been built for use in the recently completed Harbour extensions at Port Adelaide. These pumps were mounted on a large floating pontoon moored in the Cut at a position most conveniently placed dor discharge over the levee into a channel leading back to the Latrobe. The delivery line, about 36"diameter, was fitted with two flexible joints to provide for the lowering of the pontoon as the water level was reduced. As might be expected, as the water level was reduced to some 80 feet remaining, the pontoon settled on to the outer end of the jib of one of the old type coal dredgers, - this necessitated the reversal, for a day or so, of the direction of pumping to effect a float-off and adjustment of the mooring arrangements. Other large works included a new flow-under weir, re-routing of the road to Yallourn North and the building of a high level bridge on that road."