English said National would be "taking nothing for granted" because MMP elections were always close.
If National was able to get a fourth term, his preference was to work again with Act, United Future and the Maori Party.
"Together this group have provided stable and successful government which matters more now at a time of uncertainty in many parts of the world."
While NZ First was not off the cards, it was an "unlikely partner" because of its "inward-looking views" on matters such as immigration and the economy.
That was likely to mean deals in Epsom and Ohariu again to try to keep Act leader David Seymour and United Future leader Peter Dunne.
However, English said it was still too early to say what form those deals would take.
English downplayed the prospect of immigration taking centre-stage in the campaign, as it had in Britain and the United States,saying record high net migration levels were forecast to slow.
Labour Party leader Andrew Little marked the election-day announcement in Dunedin, where he spoke to an audience of about 400 last night.
He said he was ready to go to the polls in September on a platform of housing affordability, access to healthcare and quality education, safer communities and a stronger economy.
"Only by changing the Government can we do that," he said.
"We're well prepared, our teams are in place, our plans are well advanced and we'll be working hard to convince New Zealanders we can help give them a fair shot at the Kiwi dream."
Peters kept his response short, simply saying NZ First had planned around that date - "it suits us fine". However, soon after that he fired out a press statement accusing National of letting immigrants take too many low-skilled jobs.
Peters has refused to indicate whether he would prefer to work with National or Labour. Labour and the Greens paired up under a memorandum of understanding last year but have since ruled out any deals in electorate seats.
English used the platform of his election-day address to sledge his opponents, saying Labour and the Greens were "increasingly far-left and inward-looking".
He took repeated pot shots at the two parties, saying they offered nothing but a "vibe" after failing to present any new policy at the joint state of the nation addresses by Little and Green co-leader Metiria Turei.
Green Party co-leader James Shaw said his party was more prepared than any previous election and would offer "a credible, compassionate, progressive alternative" to National.
It would focus on issues such as child poverty, housing and the environment, including clean rivers.
The election date is almost exactly three years after the 2014 election, on September 20.
English said it would allow the Prime Minister to attend two important global forums - the East-Asia summit and Asia-Pacific Economic Co-operation(Apec).
Another unmentioned factor is that it falls between All Black tests on September 16 and September 30.
KEY DATES
Thursday August 17 - last day of Parliament
Wednesday September 6 - overseas voting starts
Monday September 11 - advance voting starts
Saturday September 23 - general election