Ask politely but being unable to cook probably warrants an urgent repair: https://www.consumer.vic.gov.au/housing/renting/repairs-alterations-safety-and-pets/repairs/repairs-in-rental-properties

Yep, under minimum standards a cooktop needs at LEAST 2 working burners. Write them a nice email, request that this be serviced within 14 days as its an urgent repair. Breach 'em if they don’t.