Is the taxpayer a US Citizen?

Was he an employee or was he an independent contractor?

Was income tax paid to Australia?

Yes, hes a US citizen. No taxes paid. No w2. Maybe a sub? Its not on a 1099. Just on a sheet of paper with company name. 

Taxpayer is saying with Aussie conversion rate it might not even be $600 US dollars? 

If the person/company paying him had 'control' over his work, he was likely an employee.  If that is the case, go to line 1z of the 1040 and double-click it.  Double-click again.  Go to line i4 and enter the foreign wages.

If the person/company paying him did not have 'control' over his work, you need to determine if it is a business or 'other' income.  If the work was unrelated to his 'regular' work, it is likely 'other' income in which case you enter it on line 8 of Schedule 1.  If the work was very similar to his 'usual' work, it might need to go on Schedule C.

Yes, you need to convert it to US dollars.

"with Aussie conversion rate it might not even be $600 US dollars?"

You perhaps are thinking of the 1099 issuer regulations. That isn't the perspective for your taxpayer.