English is facing plenty of challenges as National attempts to hold government in the September 23 election.
He has downplayed the impact of Peter Dunne's resignation on National's re-election chances.
Dunne, the United Future leader and MP for Ohariu, announced his shock resignation on Monday afternoon.
National had already directed its supporters to vote for Dunne, and its candidate Brett Hudson included this message on his campaign leaflets.
English denied that this put National in an awkward position, saying it was easier to correct "one letter in one electorate" than change billboards across the country as opposition parties had done in recent weeks.
"It's not a circus. Peter Dunne's made an unexpected decision, Brett Hudson is a well-known candidate, and he'll be fighting hard to win that seat."
He was not worried about the loss of a support partner, saying National still "had a lot of voters".
The National Party had no influence on Dunne's decision, English said.
"He's made his own decision. We certainly didn't give him advice."
Asked whether he wished Dunne quit earlier, he said "at least we don't have to change every billboard in the country as the Greens and Labour have set out to do and haven't yet managed".
Hudson said he would now run a full "two ticks" campaign.
"Many people have told me that they have long wanted to vote for their National candidate as well. I am very proud to now be able to seek both their votes."
