TCPILOT you have to go through an agency, you cannot work for Korean directly.
The whole process takes three days. You tend to do the medical and the sim. assessment in the first two days, (the order being dependent on the amount of free time available on the sim. programme) and the interview is done on the final day. You may even get a day free between the sim/medical and the interview. Typically you will be flown (Business class) to Seoul on Sunday and the process then begins on the Monday morning.
Your tax liability in Australia will depend on your personal circumstances, and you should seek professional advice.
There are no particular GOTCHA's in the whole assessment. The medical is thorough, but if you have any "issues" it is possible to pass provided you can show that other regulatory authorities (CASA) are happy with you. You don't mention which type, or seat you are applying for, but the sim. assessment is straightforward. They don't expect you to fly to Korean SOP's, they are looking to see that your ability matches your CV. I would advise you make maximum use of the automatics. Check carefully the required G/S angle when flying the VOR/DME approach from the FAF for 33R at ICN (it's 6% or about 3.6 degrees IIRC). And don't let down your guard just because you finish with a visual!
Good luck.