Ross, in her first public appearance in post, said the new duty could undermine the regulator’s efforts to steer companies away from focusing on capital expenditure.
She said: “It is up to Parliament to decide what statutory duties we have but our concern that a resilience duty might lead to a conversation that was concentrated on capital investment.
“This might be seen by companies in particular as a license to build, which is precisely what we have been trying to move away from with some of the innovations in our periodic review.”
Ross said a new duty was unnecessary because Ofwat was already taking steps to ensure water companies were resilient.
“We also don’t quite understand the problem this is intended to solve. Resilience is a big part of the industry, it has been for some time and we have taken that into account,” she said
The comments came during the first sitting of the Public Bill Committee, which is scrutinising the Water Bill as it goes through Parliament.
During the debate, questions were also asked why there were no measures in the bill to stop customers wilfully neglecting to pay their bills.
Rob Wesley, Water UK head of policy said there was “unfinished business” from previous legislation which could have allowed the Water Bill to require landlords to provide water companies with their tenants’ details.
Although the legislation is set to be enacted in Wales, the government in England prefers a voluntary system for landlords.
Wesley said this “simply wouldn’t work”.
Bad debt in the water industry adds an extra £15 a year to consumer bills in England.