Thames Water said the Beast from the East and summer heatwave had an impact on its ability to meet leakage targets as it reported interim pre-tax profits crashing to £67.7 million from £218.5 million a year ago.
The water company also revealed the extreme weather sent complaints soaring as it battled supply interruptions, with written customer complaints jumping to 11,083 in the half-year to September 30 from 8,242 a year earlier.
Thames Water was ordered to pay £55 million earlier this year as a penalty for missing the commitment it made to customers to cut leaks and agreed to a further £65 million in customer compensation.
The group added that profits were also affected by a move to hire more staff to help with leaks.
Earlier this year, Mr Robertson agreed to forgo bonuses for the next two years after the company forked out £120 million in compensation to customers and penalties for missing targets to cut leaks.
But under overhauled pay plans unveiled in the group's annual report, Thames Water revealed it will pay out a maximum £3.75 million to Mr Robertson in 2020 if all targets on leaks and pollution are met.
None of the 2020 bonus will be linked to financial performance, in a shift away from traditional corporate pay deals.
Thames Water also announced at the time that it would not pay a dividend to shareholders - who are mainly pension funds - this year as it focuses on improving its service to customers.