Investment broker Forsyth Barr says the long hot summer has affected Vector's operating performance.
In the electricity distribution company's third-quarter results to the end of March, the volume of electricity carried was down 2.8%, while gas transmission fell 3.2% and gas distribution 4.4%.
Forsyth Barr senior equity analyst Andrew Harvey-Green has lowered his forecast for full-year 2013 earnings before interest and tax by $4.3 million to $631.8 million.
Mr Harvey-Green says that's because of the purchase of Contact Energy's gas metering business, which he says is unlikely to be completed before the 2012 financial year ends.
He says weaker gas and electricity demand was affected by the warm summer.