How could water get into the seatbelt pretensioners of a 2019 Holden Astra?

Unless the water has come up through the floor and somehow got into the seatbelt mechanism or parts of it, the most logical answer is that rainwater has entered through an open window, run down the seatbelt webbing and into the mechanism under the seat. Failing that, if the floor and carpet has somehow got wet in the past, some water may have leached into the mechanism.

The question is: how do you know this has happened? The pretensioners only ever fire in the case of a collision, so their condition is pretty much an unknown until you need them. If, however, you’re referring to the inertia-reel mechanism that locks the seatbelt when you pull on it suddenly, then I’m sticking with the open-window-rainstorm theory, as water could definitely run down the belt webbing and into this part of the seatbelt assembly. Failing that, the moisture could be condensation that collected if the vehicle had been stored in a damp environment for some time.