Tracking huge tax bill in expenses

Hello - I was hit by a huge tax bill when I filed my taxes in Oct. About $5,500. I have a full-time job ($180k/annum) plus some income from ETF dividends and HISA. I assume ETF dividends already deduct tax at source before paying me out? Income from HISAs isn't much. It leaves me just my office and if they're charging me the right amount on my salary each month. I've also been a PR since 22 months, and I have a feeling the subsequent Medicare levy is something that work has missed on my payslips.

Is there anything else you would recommend checking?

I also record expenses each month in order to track my savings rate through the year. Do people record a tax bill as an expense generally? Technically, I should, given that I capture my take-home salary as income, but then is it better to divide the tax bill by 12 and deduct it from each month? Otherwise it looks odd to have a huge "tax expense" in just October.

Thanks for the help :)