Payroll tax is calculated on the total Australian wages paid by an employer. If all the wages are paid in one State then that State’s threshold and rate of tax would apply.
If the employee works less than a month interstate and doesn’t change his principal place of residence then his wages would be taxed in the jurisdiction where he would usually be employed.
However, where an employee might be sent interstate on a temporary assignment lasting a month or more and all his work is performed interstate then his wages would be taxed in the State where he is working.
Each State and Territory works under the Nexus Rules that were published on the 1st July 2009. There have also been Taxation Rulings on the topic.