In our tests of single- and dual-flush toilets, the ones with the top Overall Scores in our ratings were midpriced models, costing from around $200 to about $350.

Most dual-flush toilets use a drop valve, which sits underwater at the bottom of the tank. Instead of relying on siphoning it uses gravity to do the job, which means less water for every flush.

But debris often catches in the valve, causing it to leak and send water trickling all day into the pan. And up to 50 per cent of users are not sure which button is for which flush, a survey shows.