Worse election defeat

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Yes, but Ramsey and his allies were not supported by the rump Labour Party (led by Henderson at that election) - the Party expelled all people propping up the National government.

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And funnily enough Labour's sister party in Australia suffered an equally dire landslide defeat that same year, with the left of the party splitting to form Lang Labor, the right splitting to merge with the Nationalists to form the USP (which would eventually became the Liberal Party we know and, err, love today) leading to a loss of 32 of of its 46 seats.

Australia has also had some pretty outrageously one-sided defeats at a state level - just recently there was the Northern Territory obliteration of CLP, and before that the defeat of Bligh's Queensland Labor down to about six seats and the less devestating but still impressive blowout in NSW in 2011.

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True, but I think the OP was asking for the more theatrical defeats, where a party is reduced to a nub. Like, when you're going through Wikipedia and clicking "next election" and then suddenly the visceral thrill you get when you see the a government lose more than three-quarters of it seats. I view Queensland 2015 as more of a reversion to the mean - pretty embarrassing for the LNP (and the Abbott years were littered with such humiliating underperformances) that they managed to outright lose. I think the thing with state elections in Australia, is they seem heavily managerial (i.e. people view their Premier's on a more "mayor-like" level, so are more willing to reward or punish across their ideological lines that they are more locked into federally? Perhaps that's a bad interpretation.

And seeing as we've mentioned Canada, and we're talking about state elections: Canadian state elections are a good source of amusing wipeouts. (and because Canadian parties never die, the defeated parties normally hang around the ether before magically reviving themselves twenty years later).

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The Turkish 2002 general election is probably the craziest example of this. Every single party (with the exception of the moderate wing of the banned Islamist party Virtue, which coalesced around the youthful Erdogan to form the AKP, and three other MP's who had jumped on the CHP train) was kicked out of office. PM Bulent Ecevit's Democratic Left party had a 21 point swing against it getting 1 percent of the vote. His squabbling junior coalition partners: MHP and Motherland has a respective ten and eight percent swing against them. The conservative wing of Virtue, Felicity, was also kicked.  Former PM's Tansu Cillar's conservative DYP, in opposition (though dogged by corruption cases, and her shambolic period in office) just fell under the threshold. And finally, the New Turkey Party - intended to be a successor for the Democratic Left, and containing half of its parliamentary party, got even less than its beleaguered parent organisation. (Notably that election also had a wannabee Berlusconi, Cem Uzan, running a pretty deranged campaign that got about seven percent of the vote)