How do you get rid of bees which have a hive under the house and are entering and exiting in the gap under the lowest weatherboard?

These aren't bees, these are wasps. You should bite the bullet and call a pest control pro, because attacking a wasp hive in a confined space like a crawlspace can be very dangerous indeed. You should be able to have this done for between 85 and 125 dollars. If they really are bees, call a beekeeper because killing honeybees is illegal. Beekeepers will come and remove a colony for free because they keep and use the colony for honey production and crop pollination.