A report on Wednesday said former Electrical Trade Union secretary Peter Simpson knew about a cabinet vote on workers compensation changes before it occurred in 2015.
He emailed then energy minister Mark Bailey ahead of time, outlining the vote and telling the minister which option to support.
Liberal National Party leader David Crisafulli called for an investigation into how cabinet documents ended up in the hands of a union official.
He says the premier should refer the matter to the Crime and Corruption Commission if she "values integrity".
"This is an issue of cabinet documents being shopped around like there's some sort of trivial bit of information. These are cabinet documents, going to union bosses," the opposition leader told reporters on Friday.
The LNP leader said the corruption watchdog must investigate to see if other cabinet documents had been leaked to union officials.
Mr Crisafulli said union officials could still be instructing government ministers how to vote in cabinet, which was unfair to voters.
"I want to know if union officials to this day see cabinet papers and tell elected officials how to vote," he said.
"Because Queenslanders don't get to vote for union officials, they get to vote for politicians.
"Now this is a complete breach in the way the democratic process should work and anything short of an inquiry from the CCC won't cut it."
The corruption watchdog was made aware of the emails between Mr Bailey and Mr Simpson, who died in 2020, five years ago.
The CCC said in a 2017 probe that Mr Bailey had not broken any law by using his private email for official purposes, but it was in breach of ministerial standards.
"As these breaches are not criminal offences, they are not in the CCC's jurisdiction," the watchdog said in a 2017 statement.
It also said that using private emails for a official business was a corruption risk, but cleared Mr Bailey and Mr Simpson.
"The CCC did not identify any evidence to support allegations Minister Bailey had used his personal email account to engage in negotiations with the Electrical Trades Union that would amount to corrupt conduct," the watchdog said.
Premier Annastacia Palaszczuk dismissed concerns about the cabinet breach and Mr Bailey's trustworthiness on Thursday.
She said the incident had occurred five years ago and CCC had already investigated it.
"Queenslanders can trust every member of this government - every member of this government," Ms Palaszczuk told parliament.
"I'll tell you who Queenslanders do not trust: that side."
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