Ex-Liberal Dan Cregan – who quit the party late last week to contest the March election as an independent – was at midnight elevated to the Speakership after several tense and terse hours of debate ended a day of high drama during which the Opposition and members of parliament’s burgeoning crossbench stamped their newfound authority on state parliament.

They voted together earlier in the day to establish an inquiry into allegations of conflict of interest against Attorney-General Vickie Chapman stemming from her decision to block a port development at Kangaroo Island, where her family owns land.

Then former Labor veteran-turned-Independent Frances Bedford introduced a Bill to mandate the Speaker of the House must be an independent – and must quit any party affiliation to hold the chair.

The legislation was passed after lengthy debate, but the Bill still left doubt as to the fate of the incumbent Josh Teague, who was narrowly elected to the role little over a year ago.

But Labor struck again, with an eleventh-hour motion calling for Teague to be removed.

It was carried 24 votes to 22, before Cregan stood against retiring Liberal Peter Treloar for the newly vacant role, with the Kavel MP prevailing 23 votes to 21.

After presiding over his own political execution, Teague told the House: “Before I vacate the chair I wish to indicate to the House that I’ve been honoured to have been elected as Speaker of the House [and] I have endeavoured to do my very best in this role.”

“I’ve enjoyed every moment of it and I thank the House very sincerely for the privilege,” he said.

Cregan, who was set to retire from politics before changing his mind last month only to then quit the party to contest his Hills seat as an independent, said he came to the Speaker’s chair after “deep consideration”, saying MPs had “asked for a Speaker not to serve the executive but the parliament”.

“It is my sincere belief that the Westminster tradition of having an impartial Speaker is vital,” he said.

“This is a historic moment… and we may seize this moment to bring to bear more scrutiny on the executive.”

But his elevation came amid high tension, with his former backbench colleague, Liberal MP Carolyn Power, saying: “I do not know how you can do this.”

“I think what’s happening doesn’t come from a place of integrity – it comes from a place of ego,” she told parliament.

“I’m absolutely devastated.”

The Marshall Government’s majority had been whittled away with separate court cases prompting first Sam Duluk and then Fraser Ellis to join the crossbench. Duluk was since acquitted of basic assault, but quit the party after further allegations about his behaviour at a corridor Christmas party were aired in parliament.

Ellis, however, did not vote for either the inquiry into Chapman or the change to the constitution act mandating an independent speaker.

“It causes me great distress that we’ve spent over 12 hours of the precious time we have left wasting time, in my view,” he said.

“I’m greatly saddened that we’ve wasted an entire day of parliament debating a political game.”

But Labor frontbencher Tom Koutsantonis denied this, saying Teague had been “a poor selection of Speaker by the Government”, accusing him of partisanship and a lack of impartiality.

“I suspect you will have a long and fruitful career within the Liberal Party [but] they gave you the wrong job,” he told Teague.

“You shouldn’t have been Speaker, you should have been a minister… from now on the Speakership will not be a prize of either political party.

“We want something different; we want something new.”

Former Liberal minister Stephan Knoll delivered a scathing assessment, saying: “It’s now extremely clear what today was really about.”

He said Bedford’s Bill had been “trying to provide a fig leaf”.

“That fig leaf has now been removed… they are using naked power to grab what they want, to line their own pockets,” he said.

“The high-minded rhetoric belies what is just naked power and it just needs to be called out for what it is.”

He said the episode would further tarnish the image of politicians, who “do rate above used car salesmen – but only just”.

“What the Opposition is seeking to do here today is emblematic of that,” he said.

In a separate statement sent to media after 1am, Cregan said he “did not expect to be serving in this role” but that “at a time when emergency powers are being widely exercised, I hope I can bring a fair, balanced and impartial approach”.

“I also believe strongly in the principle of an independent Speakership and in the codification in our State Constitution of the UK tradition of independent Speakers,” he said.

“If the change to our Constitution passes both Houses, I believe it will be a very significant reform with lasting consequences that benefit our democracy for generations.”