The easiest solution to this problem is to open up a separate bank account, solely for GST and PAYG withholdings payments. Each week you should estimate what your liability has been for the previous week and then transfer those funds to the separate bank account. This way, when it comes time to pay your taxes the funds are there.
Late paying your ATO obligations not only can incur a General Interest Charge but can affect your ability to get a loan in the future from a bank.
If you need assistance with this matter, give Accounts Central Services a call today.