same problem. Engage HDB, plumbers, personally apply additional sealant around the joints. Nothing work. Finally, notice same phenomenon as one forumer pointed out. suspect there is a back flow into my piping system. Nothing much I can do except to flush every time the pungent smell comes up.

I got this problem too. Later realised it is WC's pan collar problem, as described by Brainstorm.

At 1st, the ID told me it's due to the neighbour below, ask me to pour bleach into the drain pipe. No use. HDB guys came, also no use. Then a contractor told me to cover all the drain pipes holes. Stuff with wet cloth and seal it as much as possible. This is to identify where the smell come from. Cover all pipes still got smell ... so the culprit must be the toilet bowl. Finally ID asked his contractor to remove and redo the toilet bowl again. Two month of foul smell finally gone!

Pouring the water fastly and forcibly will send all the smelly contents down to the sewage.

I have a feeling it might be a leak at the cement skirting around the toilet bowl, hmmm

my office in johor austin 18 has the same issue. Toilet smell like sewage. TOO SMELLY.  and its located beside the entrance. So anyone who entered our office got that smell. 

Not sure how to solve it. Contractors came also don't know how to solve.