Garage door wooden posts rotting

[SYS49152]

Could anyone advise me on the best way to tackle a rotting garage door post? Unfortunately water running down my driveway gathers easily at this point as it is a low area. I've no idea how best to tackle this as the small area would need protecting from both rain and surface run off water. Any suggestions appreciated.

BTW the other post is also rotting but not quite as bad as this one. Also the white wire is my alarm PIR wire from the house so I need to be careful with that. (I didn't install it)

 

[foxhole]

Under cut it leaving it 30mm short of the ground should slow down the rot.Impossible to tell how bad it is from a photo.
You could chop out the wet area and tamp in a little cold lay tarmac to prevent sitting water.

 

[Bouy]

Splice a new piece in. I did similar to a much worse case and then cloaked the front of the frame to hide the joint

 

[SYS49152]

Thanks to both of you for your advice. I will do just that @foxhole and thanks for the pic @Bouy