Official results announced early on Friday showed Netanyahu capturing 72 per cent of the votes, compared with 28 per cent for challenger Gideon Saar.

Earlier, Netanyahu had declared a "huge victory."

The embattled leader appeared to be on his way to a landslide victory early on Friday, giving him an important boost ahead of the country's third election in less than a year.

The strong showing by Israel's longest-serving leader gives him another opportunity to form a government following the March election, after falling short in two previous attempts earlier this year.

The victory over Saar also keeps alive Netanyahu's hopes of winning immunity from prosecution after being indicted last month on a series of corruption charges.

"A giant victory," Netanyahu tweeted early Friday, just over an hour after polls closed.

"Thanks to the members of Likud for the trust, support and love," he added.

"God willing, I will lead Likud to a big victory in the coming elections."

In a tweet, Saar congratulated Netanyahu and said he would support the prime minister in the national election.

The result of the primary will also prolong Israel's political uncertainty.

Despite Likud's strong position, the March vote was expected to leave the party and its main rival, the centrist Blue and White, unable to form a government on their own, and Netanyahu's lingering legal troubles could again scuttle efforts to form a national unity government.

Israeli TV stations, reporting partial results from polling stations across the country, estimated that Netanyahu had received 70 per cent-to-80 per cent of the votes cast.

Turnout was just under 50 per cent of the party's 116,000 members, with many voters apparently staying home because of cold, rainy weather.

Netanyahu, who has led the country for the past decade, has maintained his position atop the political right by cultivating an image as a veteran statesman with close ties to US President Donald Trump, Russian President Vladimir Putin and other world leaders.

His refusal over the last decade to make any concessions to the Palestinians was rewarded after Trump took office, as the US began openly siding with Israel on several key issues, validating Netanyahu's approach in the eyes of many Israelis and adding to his mystique.

His fortunes have nevertheless waned over the past year, after he was unable to form a government following unprecedented back-to-back elections in March and September.

His party came in second place in September's elections, leading many observers to view the vote as the beginning of the end.

In November, Netanyahu was indicted on charges of fraud, breach of trust and accepting bribes, the culmination of three long-running corruption investigations.

Israeli law does not require a sitting prime minister to resign if indicted and Netanyahu hopes that the coming election will deliver him a majority of lawmakers in favour of granting him immunity from prosecution..

But the Supreme Court is set next week to begin considering whether an indicted member of parliament can be tasked with forming a new government.

Its decision could potentially disqualify Netanyahu from leading the next government. It's not clear when a ruling would be handed down.

Australian Associated Press