wysiwyg - FTB A is on combined income and FTB B is on the second income. I found that you have to look at the everything, such as how much each income is taxed along with the FTB to see what is better overall. 1 working or 2 working. Ages ago I had a spreadsheet that I put on the varying scenarios to see what we better.
As for taxation, they should not be taxing you as a full timer. They should be taxing your actual gross pay for that pay period. Go onto to the ATO website and use their calculator to check. They cannot just tax you extra or less, the should be taxing you as per the criteria on your TFN declaration.
If you are labour hire, then use the labour hire one:
https://www.ato.gov.au/Calculators-a...alculator.aspx
Otherwise use the standard indivual one:
https://www.ato.gov.au/Calculators-a...alculator.aspx