Debate on the Public Health and Wellbeing Amendment (Pandemic Management) Bill is set to resume on Thursday but a final vote may come as late as Friday, with MPs advised parliament could sit an extra day.
The Victorian government had been confident the bill would pass after delivering amendments to secure the support of upper house crossbenchers - the Greens' Samantha Ratnam, Reason Party MP Fiona Patten and Animal Justice Party MP Andy Meddick.
But the return of Mr Somyurek - who quit the party before he was expelled for leading a widespread branch-stacking operation - after he proved his COVID-19 vaccination status, opens the door for him to cast the deciding vote.
In an opinion piece published in the Herald Sun, Mr Somyurek said he planned to vote against the bill.
The current bill gives too much power to the state government and its leader, Premier Daniel Andrews, it lacks independent oversight and "appropriate mechanisms" to scrutinise the new powers, he argued.
"I will not support this bill in its current form, and I would encourage the government to go back to the drawing board and consult more broadly," the MP wrote.
A no vote by Mr Somyurek would result in a tie, meaning the bill would have to be reintroduced in parliament's lower house.
Upper house crossbencher MP Fiona Patten said the bill will likely be defeated since the numbers would be split 19-19, which means it would fail.
"Who knew that parliament could be used for such exacting revenge on former colleagues?" the Reason Party MP told reporters outside parliament.
"This is really disappointing. We have worked very hard to get us to this point that Victoria could lead the nation with pandemic specific legislation. Now we're in a very precarious situation that we may have no controls and no regulations during a global pandemic."
Victoria's current state of emergency expires on December 15 and cannot be extended.
The bill has become a lightning rod for anti-lockdown and anti-vaccination groups, who have occupied the steps of state parliament this week.
On Monday, they gathered around a wooden gallows chanting "Freedom", "Traitor", "Kill Dan Andrews" and "Hang Dan Andrews", as an inflatable doll depicting the premier was thrown onto the structure.
On Wednesday night, Victoria Police confirmed they had spoken to the driver of the black Toyota LandCruiser which had been towing the gallows.
The 48-year-old man from Badger Creek was issued a warning for traffic offences.
