Help with Macro with spell that changes

Below, I have the following macro so I can cast a spell while targetting my tank. I have it setup to cast Void Blast however, I like for that same macro to cast Smite when Void Blast is not triggered but not sure how to do that with the below macro to cast either 1 spell or the other? Anyone able to help?

#showtooltip
/cast [@mouseover,harm,nodead] [harm] [@mouseovertarget,harm,nodead] [@targettarget,harm,nodead] SPELL

Smite automatically becomes Void Blast so just set it up with Smite.