Cut & paste from another forum:

During our trip to cape york a very strange thing happened. I placed the pottie on the ground, (it hadn't been emptied for 3 days and was packed in the back of the ute), I noticed it was a little wobbly on the ground and the sides seemed a little ballooned, so I lifted the lid, and pulled the slide lever out and boooom, a giant poo explosion. Liquified poo and urine covered my hair, face and body as well as a small amount in my mouth when I gasped. Can anyone explain why this happened. Thanks ____.

According to a number of posts, it has happened to others.

Have done similar ..... but yours sounds a bit worse, I didn't get any in my mouth. I had used too much Sodium percarbonate (napisan) & it was hot weather.

Solution as others suggested:

When fitted into seat unit a valve opens. This valve must be blocked. If in vehicle out of seat unit press in this release valve, much less dramatic.

I always run water through this valve each time I empty, as you found can be an issue if not fully clear.

Another tip:

If there are obvious signs of pressurisation, a Wet One in the bowl before opening the flap/slide is a good idea. and keep the lid closed. Procrastination, mankind's greatest labour saving device!

Acoustic insulation ARB TPMS 3xARB air compressors After cooler Air tank On-board OCD pressure air/water cleaning Additional 50L fuel Carpet in doors ABE 2x1kg Waeco 28L modified fridge Battery 4x26ah Solar 120w Victron MPPT 100/20 DC-DC 18amps 175amp jumper plug Awning 6x255/60R18

open said potty inside the caravan so nothing valuable is damaged, and the surrounding area remains unpolluted.
then swan vesta have cheap simple disposal sticks. At work
At home

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Sodium Percolate! Napisan! Surely people should be using the proper chemicals. Might as well use petrol. Everyone can spread it - Anyone can catch it. Stay home - the life you save can be your own!

Sodium Percolate! Napisan = Vanish Oxi Action