How frustrating!

So, first, I would stop putting lavender and orange around the bun’s boxes. Bunnies (and cats) are very sensitive to aromatic compounds and lavender oil is harmful to cats and rabbits over time. Lavender in plant form is OK but the essential oils are very concentrated.

It may be necessary to block access to the bun’s litter boxes from the cat for a while to break this association. I think if you could pen your bunnies overnight while the cat is in the room, and then give them free-roam while your cat is in another part of the house that could help.

Are using the pelleted yesterday’s news? It also could help to use a different litter for your cat, one that he really really likes (such as a clay litter or some other “sandy” type).

I also think it would help a lot to use a litter box screen for the buns’ box, so your cat wont be able to dig and scratch in it. I use a screen, and have a layer of hay on top, and my cat ignores the bunnies’ boxes completely. Having the barrier of the hay layer over the screen breaks the association of it being a litter box for my cat.

I’m also not sure what style of litter boxes you have, but if the cat tends to use a covered one more, make that box his box, and then use the style he uses less for the bunnies.

Lastly, make sure your cat’s box is super clean. A lot of litter box issues with cats are tied to an unclean box, so cleaning his even twice a day should encourage him to use it. If your cat is not neutered that could also be increasing his tendency to mark.

The last thought I have, if all else fails, would be to do a complete re-arrange of everyone’s set ups and try to start fresh. Move the bunny set-up to a new area in the room, get bun a brand new litter box with a screen, clean the whole room with dilute white vinegar solution. Move your cat’s preferred litter box to the area where bun’s box used to be, and block access to your bun’s box until your cat’s box habits are really well established.

Hopefully something helps! Keep us posted on if you find a solution!

Dana gave some good advice. I just wanted to add that a litter screen actually might help discourage the cat. They want to be able to dig around in the litter with their paws and a litter screen prevents this.

My cats are in my rabbit’s pen quite frequently. I think we’ve only ever had two incidents of a cat using the rabbit litterbox, once when they were young and still learning about the rabbit’s space and the other when I was unwell and hadn’t cleaned the litter trays for a little longer than normal but I cleaned out the rabbit trays first so my cat decided to use that. I keep the rabbit litter boxes inside a cage attached to their pen, the cats know that the cage is out of bounds for them which helps. I do very rarely still find them in the cage, but they know that they’re not supposed to be in there, which helps to discourage them from using the rabbit litter trays.

Perhaps setting up some sort of fencing around the rabbit tray that makes it an obvious different area that your cat isn’t allowed in could help. It takes a lot of very consistent training but it can be done. Our cats can be fussy about the type of surfaces they step on, not liking particular mats/rugs, so if your cat is like that you can use one of those surfaces in front of the tray to discourage access as well.