Smell from Toilet / Bathroom

[buckaroo]

We have been getting the occasional smell from our toilet and bathroom which smells like sewage.

We are on a septic tank (which has just been completely replaced for a brand new Aerobic model)

We are not sure whether its this (could be coinicidental) or whether its from the toilet itself? Or if its actually dependent on the weather outside?

We were told that removing the bath plug (completely) after a bath (its a stopper plug at the moment) could cause the rush of water going down the plughole that it could cause smells to come up through the plug if that makes sense?

 

[Burnerman]

The only way you'll get smells coming up from the drain is when the water traps empty somehow.....it is possible for a rush of (usually toilet) water to suck out the water from a bath or basin trap, but you can see if the trap is empty or not.
Maybe you have a soil vent pipe near thats causing the pong, maybe due to a missing air admittance valve?
John