Bill Shorten has seized on simmering voter and Coalition party room anxieties over an expanded GST to question Malcolm Turnbull's trustworthiness - even as the Prime Minister's popularity soars.

"He has a plan and he won't tell us. What is important in democracy is trust," Mr Shorten said before leading a second day of parliamentary questioning over the tax reform plans of the government.

"I think it's important that Malcolm Turnbull trusts Australians, not only to engage in a conversation, but to tell them what the government is going to do."

It came as Liberals and Nationals in the Coalition party room also aired concerns over the electoral and economic risks associated with making the cost of living more expensive if the 10 per cent tax is widened in scope or lifted to 15 per cent.

While the latest Newspoll released on Tuesday suggested fears of a higher GST debate were not showing up in Mr Turnbull's stellar approval ratings, privately commissioned research - and a welter of anecdotal evidence flooding into the electorate offices of MPs - has bolstered opposition confidence that it is on a sure thing with its anti-GST position.

As Fairfax Media reported on Tuesday, focus group testing by the experienced pollster Tony Mitchelmore had revealed voters tend to view talk of a higher GST with trepidation.

Mr Turnbull, however, continued to insist in Parliament that fairness and equity would be at the heart of any reforms of the tax system including changes to the GST.

But hostility to the concept, in the absence of any specific details, appears to be hardening - and not just on the other side of the political fence.

A series of Liberal and Nationals MPs rose in the government's weekly party room meeting to speak about the open-ended GST and wider tax reform debate.

Cory Bernardi, a Liberal conservative and known internal critic of Mr Turnbull, reportedly criticised what he said was a tendency to talk about the billions that could be raised by the GST rather than the alternative focus on growing the economy. Others also weighed in, including Nationals senator Matt Canavan and Liberal backbencher Angus Taylor.

Both warned against taking the easy option of merely hiking up the GST and stressed the need for structural reform which resulted in lowering the overall amount of tax taken out of the economy.

In a lengthy explanation, Mr Turnbull agreed that growing the economy was the best way to increase revenue.

Later in Parliament, Mr Turnbull defended his approach of keeping all options for tax reform "on the table" and again criticised Labor for trying to get him to rule things in or out.

But Mr Shorten is showing no signs of backing off, instead threatening a major escalation of the case against a GST hike.

"We haven't even begun to debate the GST campaign yet," he told reporters.

"I wouldn't predict that Australians want to pay 15 per cent more on everything.

"The government though is - they're trying to say in some sort of a dinner party way, let's talk about everything.

"Are they seriously talking about putting 15 per cent on fresh food or not? Are they seriously talking to Australians that they're going to put up the price of everything?"

Labor also produced examples of Mr Turnbull himself calling on the then prime minister Kevin Rudd to rule things out in order to ease voter fears.

Mr Turnbull said the GST was an efficient tax even if it presented problems for fairness.

"The GST itself is a very efficient tax and it is widely regarded as such and is regarded as such by just about any economist and, of course, just recently by the OECD.

"The problem with the GST, of course, is that it affects households with low incomes, lower incomes because they will spend a larger portion of their income on goods and services that are subject to the GST.

"So its advantage is its efficiency - it doesn't distort economic activity - its disadvantage is it's regressive, although studies indicate it is not as regressive as was thought years ago."

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