What you have here is innocent mistake by both parties. Essentially, mutual mistake.
The basic rule is that the money is repayable. You weren't expecting it and aren't claiming any right to it. Fair enough. The employer did not intend or owe the payment to you, it was transferred by mistake.
As a matter of interest there are examples of prosecutions of people who have discovered windfall sums mistakenly deposited into their bank accounts. If they spend it, the law says they have no "colour of right". No lawful expectation that the money is theirs.
However there is an exception which could apply in your case. If you had received the money in good faith ie. had no reason to think it was an error, and spent it honestly believing it was yours, then you can't be compelled to repay. Particularly after this length of time.
For example, you are told bonuses/holiday pay will be credited and a largish lump arrives, most people won't go into the detail.
The counterargument is that if you normally receive say $1000/fortnight, and suddenly $10,000 appears, it is damned hard to argue you legitimately expected it. Hope this helps.

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