Ask neighbours if they are experiencing the same problems at the same time.
If there is no planned maintenance or a fault or outage, it could be an external power source causing the reception difficulties, such as:
Power lines (particularly if during hot, dry, and windy weather conditions, or at night)
Street lights (when they turn on or off, or if they are broken/flashing on/off)
Equipment used by people in the neighbourhood, such as amateur and CB radios or signal boosters
An experienced local antenna specialist is needed in these cases.
Interference from power lines or streetlights should be reported to your local council or energy authority.
For other confirmed external interference, antenna installers can assist you in reporting via the ACMA digital TV interference form. (https://www.acma.gov.au/digital-tv-interference-request-investigation-form)