These are probably Heterotermes
Which are termites more interested in the fungus growing on the surface of timber such as paling fences, sleepers in retaining walls, etc.
They come from the soil and they certainly have been known to find a way inside a house through weepholes.
They seldom get into an outside monitor and the Colony Killer Termite Bait is of little use as they don’t build a substantial nest.
Probably the easiest way to kill them is to brush off the plastered ‘mud’ mixture and spray the area with a bifenthrin or permethrin solution made from a concentrate bought from your local hardware store.
You should also apply a high volume along the line where the fence touches the ground and particularly around the posts.
This is best done with a watering can so there is no droplet drift and the soil is saturated.
The residue in the soil will last for about 5-10 years. And, because you can’t be sure you’ve actually killed the nest by destroying thousands of soldiers and workers, you should inspect the area (and your house) every 3 months for the next year.
Re-do if necessary.
You can use our Termite Identification service to help you in identifying your termites.