Argentina's conservative leader Javier Milei won a decisive victory in Sunday's midterm elections, boosting the flagging reform agenda of the US-backed right-winger.

Milei's La Libertad Avanza (LLA) rebounded from a series of setbacks to take 40.84 percent of the votes cast for members of the Chamber of Deputies and Senate, according to official results based on 90 percent of votes counted.

He hailed his party's runaway victory the midterms as a "turning point" for the country and vowed to charge ahead with his agenda of shrinking the state and deregulating the economy.

Milei's LLA far outpaced the opposition in an election closely watched by jittery investors.

"Today we reached a turning point, today begins the construction of a great Argentina," the 55-year-old president told supporters at a victory party in Buenos Aires.

He promised to continue on the reform path with what he predicted would be "the most reformist Congress in Argentina's history."

Half of the seats in the Chamber of Deputies and one-third of the Senate seats were up for grabs on Sunday.

Milei said LLA had more than tripled its seat count, winning 101 seats in the lower house Chamber of Deputies, up from 37, and 20 seats in the Senate, up from six.

The centre-left Peronist movement, in power for much of Argentina's post-war history, trailed in second place with 31.64 percent.

The elections were the first national test of Milei's support since he won office two years ago on a promise to revive the long-ailing Argentine economy through a series of painful reforms.

The run-up to the vote was marked by a run on the national currency, the peso, that forced Milei to seek a bailout from US President Donald Trump, a close ally.

Washington promised an unprecedented US$40 billion package of aid, but the assistance came with a warning from Trump to Argentines that he would not "be generous" if the election did not go Milei's way. (AFP)