Malcolm Turnbull has accepted the government may need to negotiate on its proposed same-sex marriage plebiscite after Labor and others parties signalled they plan to block the popular vote.

In an interview on 3AW radio in Melbourne on Friday, the prime minister was asked several times if the changes to its superannuation package announced on Thursday meant all election policies – including the plebiscite and company tax cuts – were now on the table.

“We may have to negotiate on all of these matters,” he replied. “The one thing that is very clear is that you cannot expect to get legislation through the Senate on a take-it-or-leave-it basis unless people agree with the proposition.”

Turnbull said the plebiscite was an election commitment and the government planned to take all its election commitments through the lower house. But without a Senate majority the government would need support from Labor, the Greens or nine crossbench senators.

He said the government had strong support from the crossbench on some measures, including changes to the Fair Work Act to intervene in the Country Fire Authority dispute. “But there will be other measures where we will need to make amendments and compromises and the Australian people understand that,” he said.

Asked again about the plebiscite and company tax cuts, Turnbull said he would negotiate with senators, not in advance through the media. “We will sit down and talk and seek to do the best deal for the government, and the people, and the nation, and the parliament, with the people who have the say in the Senate.”

On Wednesday the government introduced the bill for its marriage equality plebiscite, which included $15m of public funding for the yes and no cases, despite objections from marriage equality advocates and Labor.

It is understood that the opposition leader, Bill Shorten, will recommend Labor block the plebiscite, joining the Greens, Nick Xenophon and Derryn Hinch to defeat the bill.

If Labor blocks the plebiscite the options to legalise same-sex marriage are few: the government could seek to amend its plebiscite proposal to win support, or the Coalition party room could consider allowing a free vote on legislation. Otherwise marriage equality seems unlikely to be achieved in this term of parliament.

Turnbull has not ruled out a free vote but on Thursday warned that if Labor, the Greens and others blocked the plebiscite “the resolution of the same-sex marriage issue will be postponed potentially for a very long time”.

“Important issue though it is, it is absolutely not the top of my agenda in the way it is for some others,” he said. “We’ve provided a very fair means of resolving the issue on 11 February [the date allocated for the plebiscite].”

On Thursday Tony Abbott’s former chief of staff, Peta Credlin, told Sky News some conservatives feared Turnbull planned to “take them on” in the party room to win a free vote on same-sex marriage.

She said a prime minister with a one-seat majority “will be thinking about legacy – while Labor will want the legacy of same-sex marriage under their belt – the prime minister would want it as well”.

Both conservative Coalition members, including Abbott, and MPs in favour of marriage equality have warned Labor not to block the plebiscite because no free vote would be given.

On Friday the Liberal MP Tim Wilson told Guardian Australia that Turnbull’s comments “highlight where marriage of same-sex couples now sits, it now sits in the lap of Labor, the Greens and the crossbench”.

“They’re welcome to come to the party and have a conversation about how the plebiscite – which has an election mandate – can be conducted or they can walk away from the table and have couples left at altar.”

The assistant minister for multicultural affairs, Zed Seselja, told Sky News the government would have to negotiate at times but not in a way that “compromises our end goal or our values”.

“We won’t do it by just ditching our policies.”

Seselja said a free vote in parliament would be a “radical shift” from the plebiscite the Coalition promised at the election. “Clearly they’re two very different policies – that’s the Labor party policy ... that they took to the election and lost, to put it to the parliament.”

Compromise may be difficult for Labor after Shorten warned that the plebiscite might lead to self-harm by vulnerable LGBTI youth. Labor has set out many other objections including that it treats LGBTI rights differently to other issues decided by parliament, and is a departure from the normal process of representative democracy.

But some measures that would make Labor more likely to support a plebiscite include scrapping public funding for the campaigns and making the plebiscite self-executing, ushering in marriage equality automatically if the yes vote won.

The Greens have said they will not support the plebiscite, as has the Nick Xenophon Team. The NXT cited the process’s cost and said the decision should be made by parliament.