Malcolm Turnbull has denied being caught out over dual citizenship issues after campaigning north of Brisbane without a candidate for the Longman by-election.

The prime minister was unable to endorse a candidate for federal seat while he was on the hustings after preselection was delayed by citizenship questions over two leading contenders.

The Liberal National Party has had plenty of time to prepare for a by-election, given questions over the citizenship of Susan Lamb were raised last year.

The Labor MP stood down last week after Senate colleague Katy Gallagher was disqualified by a High Court ruling.

LNP president Gary Spence told News Corp Australia there had been delays in Longman preselection due to citizenship concerns over Papua New Guinea-born Trevor Ruthenberg, a former state MP, and another candidate.

Mr Turnbull denied it was embarrassing not to have a candidate in the winnable seat, which Ms Lamb narrowly took from Wyatt Roy less than two years ago.

"The LNP will be making their selection on Tuesday," Mr Turnbull told reporters on Thursday.

"I understand they've called for nominations and they'll be received and the preselectors will make their choice on Tuesday and I'm sure it will be a very wise one."

Ms Lamb was made aware on Tuesday the renunciation of her British citizenship had been complete.