sparky,
i had this problem a few year ago and a builder pal of mine told me that the sink,washing machine,gutters should be going into a seperate pit,mainly the wahing machine becos of the detergants not breaking down the waste properly...he checked it out for me and now only have the toilet direct to the tank and all else is going to a seperate pit, i get the tank emptied once a year and all is fine...i have not had a bother since,,,,well until the worry of this septic tank charge surfaced.......