Five byelections – including two crucial marginal seat contests – will be held on July 28, Speaker Tony Smith has confirmed, in a significant delay that prompted claims from Labor of a “sneaky tactic”.

After Question Time on Thursday, Mr Smith said he had received advice from the Australian Electoral Commission that July 28 was the earliest “optimal” date for the byelections.

That was because he could not issue the writs until the commission had implemented a new nomination process to avoid another citizenship fiasco, and because there were school holidays between June 30 and July 21.

The date for the five byelections, in Perth and Fremantle, Mayo (South Australia), Longman (Queensland) and Braddon (Tasmania) coincides with the weekend of Labor’s National Conference.

The announcement sparked fury in the House of Representatives with Shadow Attorney-General Mark Dreyfus booted from the chamber as one MP repeatedly declaring the ruling “an outrage”.

In an attempt not to reflect on the Speaker, Manager of Opposition Business Tony Burke noted that Mr Smith had made the decision in consultation with the commission.

But he argued the delay was completely unacceptable, saying residents in the five electorates would be without a parliamentary representative for 78 days.

In a statement, Labor’s deputy leader Tanya Plibersek said it was a “disgraceful delay and a sneaky tactic from Malcolm Turnbull”.

“It would appear this has been deliberately designed to disadvantage the Labor Party, given our National Conference is scheduled for that weekend,” she said.

Saying he accepted the decision was now made, Mr Burke said the date selected “just happens to be on the same day as the Labor Party National Conference”.

Finance Minister Mathias Cormann said the byelections would have been held much sooner if the three Labor members had resigned earlier and Bill Shorten had not run a “protection racket” for his MPs.

The clashing dates would put significant pressure on Labor’s campaign, meaning the conference would be likely to be rescheduled.

Disputing suggestions the delay was unprecedented, Mr Smith said a byelection in Gippsland in 2008 was held 80 days from when the MP’s resignation was handed down.

“That was a single electoral event, not five caused across four states,” Mr Smith said.

Last year’s Bennelong byelection took place one day after the New South Wales school year ended, Labor figures pointed out on Thursday.

Speaking on Sky News, former Labor minister Stephen Conroy suggested the July 28 date meant the government was likely to call an early election, most likely for August or September.

Electoral Commissioner Tom Rogers was forced to front a Senate Estimates hearing immediately after the Speaker’s announcement.

“It’s not my role to set the date of byelections,” Tom Rogers told the hearing.

Mr Rogers said he had provided advice but the commission had told the Speaker it was ready to hold a byelection on whichever day was deemed appropriate.

In a fiery clash, Labor Senator Penny Wong accused Mr Rogers of providing a “carefully written letter” that provided “cover to the Speaker”.

When Finance Minister Mathias Cormann suggested Senator Wong was reflecting on the independence of the electoral commissioner, she replied: “Yes, I am.”

In a pointed remark given Labor’s recent citizenship woes, Mr Rogers took issue with any suggestion he had acted improperly.

“Several members of the House have resigned, resigned as a result of failing to follow procedures, and now somehow the AEC is being fingered as being responsible for the outcomes of this,” he said.