Opposition Leader Sussan Ley faces an increasingly emboldened conservative backbench bloc, Liberal MPs have warned, amid frustration and confusion over the timing of Andrew Hastie’s resignation over immigration policy.

Hastie resigned from the frontbench on Friday, saying Ley had told him he would not have a role in the formation of the Coalition’s immigration policy before the next election, despite holding the home affairs portfolio.

Hastie quit following a series of provocative policy interventions, including suggesting high levels of overseas migration under Labor were making Australians “feel like strangers in our own home”. On Saturday he insisted there was no imminent challenge to Ley’s leadership.

But Liberal MPs said the move had badly undermined plans to scrutinise the government in Senate estimates hearings this week, including over its management of the Optus triple-zero outages and the return of Australian women and children who travelled to Syria with members of the Islamic State group.

The Coalition plans to target the communications minister, Anika Wells, on the Optus outages and to probe the government’s involvement in the return of women and children from Syria. Prime minister Anthony Albanese disputed reports the government was organising the return of about 40 Australians from Syria in September. Some of the group have returned to Australia.

Senators James McGrath and Jane Hume, shadow attorney general Julian Leeser and shadow education minister Jonathon Duniam could be the winners from a reshuffle sparked by Hastie’s decision.

Victorian Liberal Sarah Henderson said Hastie had been “sidelined” over his views on immigration on Sunday. The shadow immigration minister, Paul Scarr, a moderate Liberal MP, is expected to lead the formulation of Coalition policy.

“All I can say about the Liberal party is the backbench is a very powerful place to be,” Henderson told Sky.

“It gives us the freedom to exercise our views, to prosecute the issues which are important to all Australia. That includes immigration. We’ve got to see dramatically lower immigration.”

The shadow communications minister, Melissa McIntosh, expressed support for Hastie on Sunday.

She said MPs of the calibre of Hastie, Hume and former frontbencher Jacinta Nampijinpa Price and Jane Hume should be given prominent roles.

“He’s done great work for our party but he’s made his decision and he’s not indicated that there’s any other intent behind it except for not being able to voice his opinions on immigration,” she said.

“It’s important to his community. It’s important to my community.”

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Ley has said Labor’s plans for 185,000 permanent migrants to be allowed to settle in Australia in 2025-26 is too high. The total number – which is split between skilled workers and family visas – is unchanged from last financial year.

Her backers argue Liberals looking for changes to Australia’s migration rules should explain their position through the opposition’s policy development process.

Ley could look to resolve the Coalition’s thorny internal debate about net zero by 2050 policies sooner than originally planned, part of efforts to calm internal ructions.

Hastie and Nationals including Barnaby Joyce and Matt Canavan have pushed for the opposition to dump its support for net zero.

Conservative Coalition MPs talked down any immediate challenge to Ley’s position on Sunday, but stressed backbenchers were allowed to speak freely on policy positions in the Liberal party.