The first full sitting week since Malcolm Turnbull became Prime Minister began with hopeful signs for an improved quality of political discourse.

Before question time, Mr Turnbull spoke powerfully about last week’s Parramatta shooting, warning that “those who try to tag all Muslims with responsibility for the crimes of a tiny minority, and convert that into a general hatred of all Muslims are undermining our national interest”.

It was an appeal to national pride at being the “most successful multicultural society in the world”, rather than a dog-whistle to whip-up latent racism.

His answer to a question on the emotive issue of penalty rates was equally rational.

• Medicinal cannabis set for Vic
• PM keeps his enemies close
• New anti-terror laws to target radicalised teens

He presented cases of enterprise bargaining agreements, negotiated by unions, that had scrapped penalty rates but increased pay packets overall.

All encouraging stuff – and reasoned argument.

Mr Turnbull promised no less when he became leader in September.

But just how far can Mr Turnbull’s ‘age of reason’ go, particularly with the right of his party still angry and still despising his small-‘l’ liberal views?

One early test will be the issue of medicinal cannabis – the subject of a private members bill introduced into the Senate and co-sponsored by Liberal, Labor, Green and Independent senators.

Victorian Premier Daniel Andrews brought cannabis back into the national debate last week. Photo: Getty

The cross-party group of senators – Greens leader Richard di Natale, Queensland Liberal Ian Macdonald, Tasmanian Labor senator Anne Urquhart and Liberal Democrat David Leyonhjelm – fronted cameras in the Senate Courtyard on Monday to push the issue along, joined by two lower house MPs, Liberal Sharman Stone and Labor’s Melissa Parke.

And their arguments were thoroughly rational.

Medicinal cannabis has been rigorously trialled and found to be effective for certain acute conditions (though many more require such testing), such as types of epilepsy, severe muscle spasms, and as pain and nausea relief during cancer treatment.

It is so effective, that adult patients, and the parents of young patients, are now routinely buying cannabis on the black market – effectively becoming ‘criminals’ to gain access to effective medicine.

One such parent, Lucy Haslam, was with the parliamentarians to explain how her son Dan, who died in February, had endured days of vomiting during treatment for bowel cancer, before successfully treating himself with the illicit drug.

She explained that “many thousands of people” want legal access to the medicine he had to obtain illegally.

But can the Turnbull government get behind this?

If they do, it could pass both houses before the summer recess in December, meaning many suffering patients could have safe, imported, medicinal cannabis in their hands before the end of 2016.

Thereafter, the plan would be to licence secure growing facilities in the same way that medicinal opium poppies are already grown in Australia for the manufacture of opiate medicines.

There’s even the prospect of Australian cannabis being exported to other nations.

As Senator Di Natale pointed out, the potency and variety of strains available on the black market are unknown quantities for patients who break the law to escape pain or debilitating conditions.

What might convince Mr Turnbull’s more conservative colleagues to get behind this legislation are broader issues around the economics, and law-enforcement aspects of the drug.

Selling prescribed cannabis under strict guidelines – remembering that many dangerous drugs are prescribed by GPs in Australia under such guidelines – would reduce demand for illegally grown product.

The Victorian government, which already has plans to seek federal approval for its own medicinal cannabis programme, thinks a monthly dose could cost about $50 to $60 – a price point that provides little incentive for competition from illegal growers.

Moreover, patients would no longer be a burden on the law enforcement and judicial systems for criminal behaviour.

The MPs sponsoring the bill have decided not to foreground their own views on the decriminalisation of recreational cannabis – their only goal is to give patients access to a drug that is now part of the medical systems of Canada, the Netherlands, and parts of the US.

These are rational arguments, but they must do battle with emotional prejudices in the community – views that see recreational cannabis use as the first step towards harder and more dangerous drugs.

That’s why Monday’s debate in parliament will be heartening to the cross-party MPs.

If Mr Turnbull can tackle fear or hatred of Muslims, or outrage over ‘stripping away penalty rates’, head on, perhaps legalising medicinal cannabis won’t be so hard after all.