The Commerce Commission says competition for the supply of key building products is not working well as it could.
The watchdog has just released its draft report into competition in the residential building supplies market.
It says it is too slow, costly and uncertain to get new products accepted and that must be improved.
The Commission also raised concerns about rebates paid by established suppliers to merchants.
It recommended introducing more compliance pathways and making it easier for new supplies to be introduced.
RNZ business reporter Nicholas Pointon spoke to Corin Dann.