I happened to pay my mobile bill late, thanks in advance to all those people who are about to say 'pay it on time and it won't be a problem, cheers got that'.
The bill was paid before the next one arrived however obviously not soon enough to escape the dreaded late fee.
What I noticed shocked me $18.40 late fee on a $59.95 account, that's 30.6% interest per month, surely this couldn't be considered reasonable cost recovery on such an amount - they make pay day loan companies look like the good guys.
If you were a perpetual late payer you'd pay $220.80 just in late fees per year.
So I've solved the problem by moving to Skinny Direct and while in a round about way Spark will still get some money they've halved their ARPU so well done Spark.