Basically, this calculation is relate to this blog; I somewhat remade it based on Therefir's suggestion.
[Calculation 1]: This calculation is Therefir's suggestion for destroying Australia; credit to Therefir on this calculation:
Diameter of Australia = 4000 km
Explosion radius = 2000 km or 2000000 m
W = R^3*((27136*P+8649)^(1/2)/13568-93/13568)^2/1000000
W is yield in megatons of TNT, R is radius in meters, and P is pressure of the shockwave in bars, the standard overpressure is 20 psi or 1.37895 bars.
W = 2000000^3*((27136*1.37895+8649)^(1/2)/13568-93/13568)^2/1000000 = 642947485.59 Megatons of TNT, or 642.95 Teratons of TNT (Large Country level+)
[Calculation 2]: This calculation is for destroying Continental United States; I found that that Australia isn't largely considered a proper continent on its own worldwide. So, I chose the Continental United States since it would approximate a small continent. This case could also apply for a similar feat in the LN since Leon can sink a continent larger than Australia.
Diameter of United States (contiguous) = 2800 miles [1] or 4506.163 km.
Explosion radius = 2253.0815 km or 2253081.5 m
W = R^3*((27136*P+8649)^(1/2)/13568-93/13568)^2/1000000
W is yield in megatons of TNT, R is radius in meters, and P is pressure of the shockwave in bars, the standard overpressure is 20 psi or 1.37895 bars.
W =2253081.5^3*((27136*1.37895+8649)^(1/2)/13568-93/13568)^2/1000000 = 9.1921e8 Megatons of TNT, or 919.21 Teratons of TNT (Continent level)
New:
[Calculation 3]: This method use a method proposed by Ugarik where Antarctica is used as basis.
Area of Antarctica = 5.483 million mi² = 1.42e+13 m²
R = (Area/pi)^0.5
R = (1.42e13/3.14)^0.5 = 2.12e6 m
Explosion radius = 2.12e6 m
W = R^3*((27136*P+8649)^(1/2)/13568-93/13568)^2/1000000
W is yield in megatons of TNT, R is radius in meters, and P is pressure of the shockwave in bars, the standard overpressure is 20 psi or 1.37895 bars.
W = 2.12e6^3*((27136*1.37895+8649)^(1/2)/13568-93/13568)^2/1000000 = 7.65e8 Megatons of TNT, or 765 Teratons of TNT (Continent level)