New Democracy was leading with 40-44 per cent of the vote, with the leftist Syriza trailing at 16.1-19.1 per cent, the exit poll conducted jointly by six polling companies for Greek TV stations showed.
Sunday's vote is the second in the past five weeks, as a first poll on May 21, held under a different electoral system, failed to give a single party absolute majority in the 300-seat parliament.
The system used in Sunday's election gives the leading party bonus seats depending on voter support.
In the May poll, New Democracy held a double-digit lead of 20 points ahead of Syriza, which ruled Greece from 2015 to 2019.
Opinion polls in recent days had shown New Democracy winning with about 40 per cent of the vote, with Syriza headed by Alexis Tsipras on about 20 per cent, roughly similar to the May 21 poll.
Mitsotakis, who was prime minister from 2019 until stepping down in favour of a caretaker prime minister following the inconclusive May vote, has vowed to push ahead with reforms to rebuild the country's credit rating following a debilitating debt crisis which wracked Greece for a decade.
A former banker and scion of a powerful political family, Mitsotakis has promised to boost revenue from the vital tourist industry, create jobs and increase wages to near the European Union average.
The COVID-19 pandemic and a deadly rail crash in February also exposed the shortcomings of the health and public transport systems.
But a cost of living crisis and economic hardship have more recently topped voters' concerns.
"I expect a lot (from the new government)," pensioner Giorgos Katzimertzis told Reuters.
"The main thing is the health system, the economy, so we can live (decently) because things are difficult. I am a pensioner, I was on the fire brigade, and now I don't have a dime."
Sunday's election was held in the shadow of a migrant shipwreck this month in which hundreds are feared to have perished off southern Greece.
One of the worst such disasters in years, it has exposed the parties' divisions over migration.
The shipwreck disaster dominated campaigning in the run-up to this election.
Rescuers found 104 survivors and recovered 82 bodies but up to 750 people are thought to have been packed on the ramshackle vessel heading from Libya to Italy.
The boat had been shadowed by the Greek coast guard, which said occupants refused all offers of help.
Mitsotakis, whose administration has taken a tough line on migration, has blamed "wretched traffickers" for the disaster and praised the coast guard for rescuing people.
Tsipras has questioned why the coast guard did not intervene earlier.
