Do I need an electricity converter for my 2017 MacBook Air and iPhone Xr chargers in Australia?

No, you do not need to adjust the electrical requirement for these Apple devices. The power supplies and the AC adapters auto-switch to leverage any common global voltage between 100v-240v.

For example:

iMac - iMac Technical Specifications - Apple

❝Electrical and Operating Requirements

• Line voltage: 100–240V AC
• Frequency: 50Hz to 60Hz, single phase
• Operating temperature: 50° to 95° F (10° to 35° C)
• Relative humidity: 5% to 90% noncondensing
• Operating altitude: tested up to 16,400 feet (5000 meters)❞

MacBook and iPhone chargers can use the same line voltage.

The Apple charger is typically multi-voltage, as correctly mentioned above. Check the labels on yours.

Some of the Apple chargers have so-called duckheads that can be swapped:

• Using Apple power adapters, cables, and duckheads with Apple products - Apple Support
• About the Apple World Travel Adapter Kit - Apple Support

if your charger has one, the duckheads can either connect directly to the power contact pins, or can connect to a power cable with the contact pins.

For the smaller-wattage chargers, you can obtain a replacement in Australia, and can usually use a shaving adapter in a hotel room, or using travel kit adapter. But given the cost of some of the travel adapter kits, buying a sufficient-wattage charger and the correct duckhead is probably the cheapest.