No. In general, your employer cannot change your contracted working hours without your consent.
If your employer decides that they no longer want anyone to perform your current role, and restructure the position into a lower hours role, then your first position may have become redundant and a redundancy payment may be applicable.
However, during the COVID-19 crisis, you may also choose to agree to a temporary reduction in your working hours in order to help avoid a redundancy or being stood down. If this is the case, you should make sure details such as the review and end dates for the new arrangement, and any rights to return to your regular hours, are clearly set out in writing between you and your boss.
Note: some awards are being varied in response to COVID-19, including in relation to working hours. This is a developing situation as changes are being made on a case by case basis. You should check the award that applies to you. If you are unsure which award applies to you, you can check on the Fair Work Ombudsman website.
If your employer is taking part in the Jobkeeper scheme, then they will have additional powers to ask you to change your normal working hours. If this is the case, refer to the JobKeeper section of our Know your work rights during COVID-19 FAQs.