The answer really depends on whether, when the person does work, the wages earned vary from period to period.
If the answer is yes, the rule is given by sub-section 57(3) – in which case you start at the termination date and work backward, looking for weeks (any 7 consecutive days ) where the person worked. Add the wages earned in each week worked and stop when you get to 13 (i.e. skip over any unworked leave). Then divide these total wages by 13 to get an average.
If the answer is no, then sub-section 57(1) applies and the termination pay owing is what the person would have earned for regular hours during the notice period.