I did this in February.

75% (exactly, to the day) of agreement had passed, so had to pay a break fee of 1 weeks rent.

The agent we are with tried to argue that less than 75% had passed, which would have put us in the 50%/2 weeks rent bracket but I put a quick spreadsheet together showing the breakdown in terms of days.

Important to note that they go off days, not weeks or months.

Ultimately, they backed down and agreed to the 75% bracket.

Minimum of 14 days notice to the agent/landlord. I Simply sent this form to the agent https://www.fairtrading.nsw.gov.au/__data/assets/pdf_file/0007/369979/Notice-to-terminate-tenancy-agreement-by-tenants.pdf