Waste coming up shower trap

Shower trap not properly seal,or a conection on the waste pipe not secure enough.

Your sink drain is higher than your shower drain.
When water is going down the sink drain it hits a blockage with nowhere to go it backs up and it flows through the drain to your shower.
You have a blockage in your drain pipe T connecting your sink drain and the shower drain.
That's the only thing I can think of. You can try to snake it yourself or call a plumber.

The smell might be sewage or just stagnated build up of a mix of things . try to use a plunger to make sure your shower drain is not blocked in any way , to use a plunger correctly you will need to block the drain hole and fill the shower with hot water and remove what you have covered the drain with and start plunging while the water is running down the drain , this gives maximum effect while using your plunger , some material may want to expel out the top of the drain while other material will be forced down and along the waste pipe after you have done this pour a large cup or two full of liquid bleach down the shower drain and see if this clears the problem ,because most sewer systems are independent from the waste water of sinks, showers and washing machines. Also when you flush the toilet try adding a bucket of water at the same time as you press the flusher , this will force more water than usual down your toilet helping to keep the pipes fully open , but if you notice that when you flush and add the bucket into the bowl at the same time that the water does not escape freely and seems to back up , this can be a sign that there may be blockages in the sewer pipes and you might try doing this extra water flush a few ,more time hoping you will clear your pipes adding some Landry washing powder to the bucket can have a effect on moving anything along and hot water is beneficial as well . But I think you will find that the shower drain is what is causing the odour pouring a mix of vinegar and baking soda down the drain as a regular maintenance job keeps them odour free and unblocked .

Yes water can back up in a drain and flood out lowest opening, which would be a toilet flange at floor level.
If tub is draining fine, then clog is probably near the toilet, but below floor flange level, and therefore not inside the tank ... judging from your description.
If you are in a house, go outside and see if there is a vent stack on the roof directly above toilet. If so, you can run a snake down from there.
You can also run a snake down through "s" shaped toilet trap and enter the pipe that way.
Another option is to remove the toilet and run snake directly down hole, and then reattach toilet.

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need more info

your waste pipe is partially blocked, clean trap and drain.

We have a maserator in en-suite which serves the toilet & shower, We moved in to house 6 years ago and it was already here. After having guests stay a couple of months ago the maserator hasnt worked properly. When we flush the toilet, the water rises and whilst the cistern is filling with water, there is a humming noise for a couple of minutes, then the water rushes back down and there is a clonk and the water bubbles back up through the shower plug hole.Any suggestions as to what this problem is & how to fix it please?

sounds like there may be some blockage in pipes. try to run a long snake through both drains or run all fixtures in bathroom and try plunging vigorously in tub and shower while running water. if nothing still, it may be possible that your underground plumbing is back graded or incorrect fittings and plumbing installation.

Yes the weight of someone in the tub plus the water can push the tub down far enough to cause the leak.
Unscrew the flange in the tub that holds the drain to the tub. Get some plumbers putty and make a bead to fit between the tub and flange.
Make sure the rubber gasket is clean ..use a bit of plumber's putty on this as well. Get an assistant to stand in the tub while you snug things up.
Snug it up tight.... replace if the threads are stripped as you said.
Plus if your house is less than a year old, I'd talk to the contractor/builder if possible about warranty work...most reputable builders will cover their work for the first year.

The only way to reduce water waste would be to have a plumber install smaller pipes coming from the water heater to each individual point of use. The normal way of plumbing a house uses a large pipe that has smaller pipes that go to each sink, tub, shower, etc. The larger the pipe, the more water is in it and the longer it will take to get hot water to where it is going. An electric tank won't save you from wasting any water unless you have a separate hot water circulation system installed. This would keep the hot water drawn out to the far ends of the plumbing system and allow the hot water to reach it's destination alot sooner. However, keeping a tank full of water hot is not economical and a circulation system basically turns your pipes into a radiator causing you to waste electricity. I know it may seem that you are wasting alot of water waiting for the hot water to get there, but in the long run you and the environment are coming out ahead due to the saved energy and efficiency of the Rinnai unit. Also just in case you were thinking of it, a circulating system is not compatible with a Rinnai water heater.