Find pattern for Check Digit calculation

I need to solve the calculation that produces the check digit for BPAY payment numbers. I've found multiple references, all of which state a slightly different method to calculate it, but none of them give the same results as the test data I have. I've been told it's to use Modulus 10 Version 5, or the Luhn algorithm to calculate it, but as mentioned it does not, so it must be a custom algorithm. The start of the number is the biller ID so I am not sure if that needs to be taken into account or now, but here are some examples and their check digits. Upon solving these I will provide some more test data to verify. 1018 5122 | 9900 4184 | 6 1018 5122 | 9900 4185 | 9 1018 5122 | 9900 3887 | 4 1018 5122 | 0020 4935 | 1 1018 5122 | 0020 4874 | 5