Anyone who knows anything about maths knows that when you have to sum a series of numbers that might have more decimal places than you want in the result, the decision on whether to round before or after the addition makes a difference.
Let's take the simple example of adding 1.4 and 2.3 for a result that we want to be an integer. If we round first the result is 3 (1 plus 2), if we round after the result is 4 (3.7 rounded up).
It even matters if what you always do is round downn (or take the integer part). Now add 1.4, 2.3 and 3.6. If we round (or truncate) first the result is 6, if we round second the result is 7 (7.3 rounded down).
The story today about Telstra putting amounts to three decimal places (of dollars) on bills is therefore interesting. What isn't clear is whether this reflects the way they've always calculated. It also isn't clear whether any of their charges mightn't have relevant fourth decimal places.
It appears from the comments that the issue arises from adding the 10% GST, but the problem goes away if GST is added on the total rather than the individual charges.
But the best hoot of all was the final comment attributed to the Telstra spokesman;
It is also important to note that the 'Total Due' is rounded to two decimal points.
One would hope so - the industry has been bedevilled enough with customer stories of bills for $0.00 - imagine getting a bill for $33.333 and paying $33.33 - and then getting the wonderous bill for $0.003. That would set a new benchmark for telco stupidity.