In Australia if you are registered for GST (Goods and Services Tax), which is optional if your business income is under $50,000 a year but mandatory if it goes above that, then you are required to calculate the deemed income from any services that you provide for which you receive any form of non-cash remuneration. The GST of 10% is then payable on that income that you didn't receive and needs to be paid at the end of the quarter.
So if you wanted to be able to claim the $100 for donated services at the end of the year and you are registered for GST then you need to declare a GST inclusive income of $100 for the quarter and pay the $9 tax on it. Then when you lodge your tax return in July you would specify $91 income and $100 deduction and claim the tax deduction for the donation getting you back about $3 of the $9 paid.
As this places you in a worse tax position than if you were able to ignore the whole thing, people would try to ensure that any pro bono work like that is done via an entity not registered for GST (so if you have a partnership or company registration you'd do the work as an individual). Then since no money has changed hands and there is no GST implication requiring the calculation of deemed income you can just ignore the whole thing.
It is only where there is a deemed GST liability where there would be any need to mention the transaction for tax purposes at all.