Hi @Jdarby,
A warm welcome to the Bunnings Workshop community and thank you for your question.
You'll want to start by removing all the existing silicone and cleaning up the gap, removing any stones or grout that are sitting loose in the gap.
The best way to remove silicone is by cutting it away from the surface beneath with a utility knife. Make sure you wear gloves, long protective clothing and eye protection to prevent any unnecessary lacerations.
Once all the silicone has been removed and the gap has been cleaned out, you'll want to use a white pool silicone such as Maxisil 310ml P1 White Pool Silicone or Sikasil White Premium Swimming Pool Silicone Sealant to fill the gap.
Depending on their width, you might need to push some Abelrod into the gaps to act as a backing for the silicone. This is worthwhile as it will greatly reduce silicone wastage within the gap.
You can then mask off the gap with Painter's Tape before applying the silicone. Check out How To Silicone Seal a Bath Tub for an example of this.
You'll then want to apply the silicone along the gap ensuring the bead is full enough to cover it with no gaps or entry points for water.
You can then strike off and smooth the silicone. For wide beads of silicone, I like using a Stud Adhesive Applicator Knife as it has a nice round head and a long handle, but if you don't have one or a similar tool, the Paint Partner 4 Piece Silicone Scraper Set has a nice, rounded scraper that could be used.
Once smoothed and excess silicone is removed, pull the tape off so you get nice clean lines.
Make sure you have a bucket of soapy water and plenty of rags handy for clean up while you work. It is much easier to remove silicone while it is wet than it is after it is dry.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
