A one third drop in price in a matter of hours - there's probably an example somewhere but I can't recall any prior examples of anything major (eg major indices, oil, gold etc) losing quite that much value in a matter of just hours.
Looking at some other impacts though (with the discussion here being both physics and finance -
skip to the last paragraph if you just want the financial conclusion):
Energy content of a barrel of crude oil is about 6 GJ. It varies a few % either way, not all crude oil is exactly the same, but it's roughly 6 GJ per barrel. Some is 5.8, some is 6.2, etc but none of it's too far from that.
Oil is currently trading at $27.75 and the AUD is at about 65 cents which gives an AUD oil price of $42.50 per barrel or just over $7 per GJ.
Looking at the Australian domestic (excl WA and NT) market, natural gas has been mostly in the range of $5 - $6 in recent days. A year ago it was more like $10 - $12 but, and this is the point, you won't generally see gas prices exceeding oil prices at least not for long.
That's due to the reasons I've previously mentioned. LNG prices linked to oil prices under contract is a big one. Industrial users able to switch between fuels is the other.
Note in this context that there's more than one gas price globally and in this context I'm referring to seaborne LNG and the onshore domestic market in countries (including Australia excl WA and NT) where the LNG price sets the domestic price. From a trading perspective I'll draw attention there to the fact that the USA is not in this situation, the gas price there is driven by other factors primarily.
Gas costs more to transport, simply because it's a low density gas and to move it you either need pipelines and compression stations along the way (which are basically just pumps of a sort but they use quite a bit of energy to operate) or you can turn it into liquid at -161 degrees C. All that takes energy though and has other costs when compare to oil that can be stored in a simple low tech and low cost tank. Hence the general case of gas selling at a discount to oil.
Bottom line is that oil at ~USD 28 per barrel supplies energy at AUD 7 per GJ and that's not far from the AUD domestic market gas price of $5 - $6. It's going to put downward pressure on that gas price and thus on Australian gas producers.
For some users, those paying higher pipeline costs, gas would need to come down toward AUD 4 per GJ to match the current oil price whilst for most it is capped at no more than AUD 6 per GJ with oil at the present price.
This situation may also put some pressure on bulk pipeline owners. If they want to keep gas going through them then ultimately the price at the customers' end needs to be competitive. A contract that the customer pays $x per GJ transported only works if the customer still wants gas transported.
Looking at thermal coal, it has been trading at about USD 65 per tonne or AUD 100 per tonne recently. As with oil the energy content varies a bit, not all coal is chemically identical, but for thermal coal the official specification is 26.35 GJ / tonne.
So coal at USD 65 is equivalent in energy terms to oil at USD 14.77 per barrel. Or in AUD it's equivalent to oil at $22.70 or gas at $3.77 per GJ. That sounds good but I'll add some caution in that there's two issues which complicate it.
First, coal is more costly to handle. There's ash to dispose of, there's a greater need to control pollutants, operation requires more physical plant maintenance and staff, etc. Coal needs to trade at a discount to oil for that reason since low price relative to oil and gas is ultimately the only reason there's any serious market for thermal coal in the first place.
Second, in some uses there's a significant efficiency difference. Worst case using oil or gas in a boiler gets the same efficiency as using coal. Best case gas or oil gain a significant advantage and that's especially so in power generation in situations where it's a purpose built gas-fired power station competing in the same grid against a purpose built coal-fired one. Gas does have an efficiency advantage there such that it skews the break even fuel prices.
There's also a third level impact. In short cheaper oil and cheaper gas, and pressure on the coal price, is likely to push wholesale electricity prices down in many countries including Australia. That then has impacts for power producers using other resources, eg wind, hydro, solar, brown coal (which is not a traded market) or nuclear. They get zero reduction in costs but the value of their production drops unless they're very well hedged.
Long story short = oil at USD 27.75 is going to put pressure on the price of gas at current levels and it wouldn't need to fall too much further to squeeze the thermal coal price too. Oil at this price is going to put the entire energy sector, gas, coal and electricity as well as oil, under price pressure.