Where it goes wrong
Most cross-border freelance work performed in Australia for an overseas client is a GST-free export of services under s 38-190 — not "out of scope". You don't charge GST, but the income still counts when you tot up your $75,000 threshold.
The reverse mistake — input-taxed
A landlord with $80,000 of residential rent doesn't have to register: residential rent is input-taxed, and input-taxed supplies don't count toward the $75,000 threshold. But mixed portfolios (some commercial rent, some residential) need a careful split — the commercial rent is taxable and counts.
Cross-references
Test your registration position in the GST Registration Threshold Calculator and use the GST Calculator for BAS prep once registered.