Hi @fixmystuff,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
Start by taping around the light switch with some masking tape to protect it and stop any plaster from getting on it.
You'll then want to use a utility knife to widen the crack by cutting a V-shaped groove along the length of the crack. This is because the crack is not really wide enough to take any filler, and you need to increase the surface area that your filler will be in contact with to ensure it sticks well. Make sure you wear protective clothing and gloves while doing this to help reduce the risk of lacerations.
Once you have cut your groove along the crack, apply mesh tape like this Builders Edge 50mm x 20m Fibreglass Builders Joint Tape over the full length of the crack.
You'll then use a joint knife to apply a thin coat of Multi-Purpose Joint Compound to completely encapsulate the tape and fill the groove behind it. You'll want the majority of the plaster in the middle of the joint, feathering it out as you move further away from the joint so that there is no discernible difference in height between the new plaster and the plasterboard. A wider application of plaster makes this much easier, so make your plaster joint around 200mm wide.
Once the plaster has dried, grab a sanding sponge and give everything a sand, focusing on making it smooth and removing any height differences between the new plaster and the old plasterboard so they blend together seamlessly.
After sanding, brush the wall with a soft bristled brush to remove any plaster dust, then prime and paint.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
