This is a hot water cylinder or boiler.
Capacity: 135 Liters (approximately 30 Gallons). This is a standard size for a small-to-medium household (typically 2–3 people).
Heating Elements: You can see two grey circular caps with yellow surrounds. These house the immersion heating elements (specifically "Backer" brand, as noted on the caps).
Top Element: Usually acts as the "boost" or primary heater to quickly provide hot water for the top of the tank.
Bottom Element: Typically used to heat the full volume of the tank, often during off-peak hours.