Turnbull's conservative coalition has trailed the opposition Labor Party in 28 straight Newspolls, published by The Australian newspaper, while his standing as preferred prime minister has dropped to its slimmest lead over Labor leader Bill Shorten.
"The election will be next year, I can assure you. There is no plan, intention or consideration given to doing anything other than having an election at the usual time, which will be in the first half of 2019," Turnbull said at a business summit in Sydney.
Turnbull defended his government's track record, saying it had delivered on its commitment to boost economic growth with more than 403,000 new jobs created last year.
His popularity has fallen amid criticism of his handling of an affair between former deputy prime minister Barnaby Joyce and a member of Joyce's staff. Joyce was forced to resign last month as leader of the National Party, the junior coalition partner with Turnbull's Liberal Party, and now sits on the backbench.
The coalition holds a one-seat majority in parliament, rendering the conservative alliance vulnerable to instability.