Just cancel the month to month plan as soon as you get the phone, and the first months fee should be waived.
T&C's for Google Home Mini offer:
Google Home Mini Offer
Personal use only. Business customers must have an ABN/ACN. Bonus Google Home Mini (RRP$79) available to approved customers who sign up or upgrade to any Red Cap Plan, Red Plan or Red Plus Plan, with the Google Pixel 3a on a Mobile Payment Plan. Excludes rate plan change. Max 1 Google Home Mini per Eligible Plan. Please see your Eligible Plan details for more information. Total min cost, min monthly spend, exclusions and terms and conditions apply for Eligible Plan. Not for commercial or resale purposes. Offer not transferrable or redeemable for cash. Not available in conjunction with any other offer, and offer is subject to change. Android is a trademark of Google LLC.