Both the government and opposition have tempered expectations of winning Saturday's races for the traditional Labor strongholds of Ipswich West and Inala, which was held by former premier Annastacia Palaszczuk.
The Queensland state election is set to be held in October, with an opinion poll released on Friday indicating Mr Miles faces defeat.
The premier's rivals the Liberal National Party led the two-party preferred vote 54 per cent to 46 per cent in the poll published by News Corp.
However, Mr Miles will have a better idea of his Labor government's election hopes in Saturday's polls.
People line up to vote at Forest Lake state school in the seat of Inala in Brisbane. (Jono Searle/AAP PHOTOS)
Mr Miles told reporters he expected swings against the government "in the double digits".
"It's usual to expect swings against governments in by-elections," Mr Miles told reporters at Ipswich High School on Saturday morning.
"That's what we nearly always see and so that's what we're expecting here."
That was echoed by Prime Minister Anthony Albanese, who told Nine's Today program he also expected a swing away from the Labor government.
However, Opposition Leader David Crisafulli warned supporters to be "realistic" because Labor currently holds Ipswich West by 14.3 per cent and Inala by 28.2 per cent.
"If there is a swing towards the LNP of anything north of about three to four per cent there will be rumblings in the Labor Party," he told reporters in Brassall on Saturday.
About a third of voters had already made up their minds and submitted their decision before election day across both seats, according to the Queensland Electoral Commission.
Opposition Leader David Crisafulli came out to back the LNP's candidate in Inala, Trang Yen. (Jono Searle/AAP PHOTOS)
History will be made by whoever wins at Inala, as it will mark the first time the community is not represented by a Palaszczuk family member.
Contested for the first time in 1992, the electorate was initially represented by Ms Palaszczuk's father Henry.
He went on to become a senior minister before being succeeded by his daughter who resigned in December.
It's considered Queensland's safest Labor seat but Ms Palaszczuk took no chances on Saturday, stopping by a polling booth in the electorate to show her support.
An Ipswich West by-election was triggered by Labor MP Jim Madden departing in January.
Annastacia Palaszczuk and her father Henry held the seat of Inala since its formation in 1992. (Jono Searle/AAP PHOTOS)
He will run for Ipswich council election in Saturday's Queensland local government elections.
Since 1992, the average swing against Labor at by-elections when they are in government is five per cent.
Margie Nightingale is Labor's candidate for Inala and faces the LNP's Trang Yen.
Wendy Bourne is running for Labor in Ipswich West against the LNP's Darren Zanow.
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