Malcolm Turnbull has rejected suggestions the date chosen for five upcoming by-elections was a political ploy aimed at clashing with Labor's national conference.
The prime minister said it was outrageous for Labor to infer the Australian Electoral Commission's advice to speaker Tony Smith was designed to advantage the coalition.
"The electoral commission is utterly impartial and non-partisan," he told reporters on the NSW Central Coast on Friday.
Electoral commissioner Tom Rogers said the "optimal" July 28 date was chosen to avoid school holidays and allow candidates to comply with new regulations to declare they are not dual citizens.
But senior Labor figure Anthony Albanese isn't buying the explanation, arguing there were other dates which wouldn't have clashed with his party's conference.
"On what basis was that chosen as the optimal date? That's what we want to know," Mr Albanese told ABC radio on Friday.
"The circumstances here are red hot."
Mr Albanese accused Mr Rogers of giving confused evidence to a Senate estimates hearing over how he landed on the date.
"It is incomprehensible to me why it is, that this one day of the term, five by-elections have been called," Mr Albanese said.
There has never been five by-elections held on the one day in the history of Australian elections.
Four were triggered by resignations of MPs due to their dual citizenship and one MP resigned for family reasons.
Mr Turnbull blamed Opposition Leader Bill Shorten and his MPs, for denying the legal reality the High Court would rule them to be ineligible.
"Finally, the law caught up with them," Mr Turnbull said.
Labor national president Mark Butler confirmed the party would not hold its conference on the same day as the by-elections, with its executive committee to meet on Friday.
He is outraged at the "purely political" date which he says is a "disgraceful indictment" on the Turnbull government.
"It is quite unchartered, unprecedented territory for Australian democracy," Mr Butler told the ABC.
