Caleb  (left) and Samuel Courneloup outside the High Court in Canberra. Photo: Andrew Meares

Caleb (left) and Samuel Courneloup outside the High Court in Canberra. Photo: Andrew Meares Photo: Andrew Meares

The High Court will today consider whether a bylaw restricting street preaching is unconstitutional in what observers say is an important test of free speech protections.

Christian preachers Caleb and Samuel Corneloup were prosecuted under a bylaw for preaching in Adelaide's Rundle Mall.

The bylaw requires a person to obtain a permit to ''preach, canvass [or] harangue'' or distribute printed material.

The brothers challenged it, and last year the Supreme Court of South Australia found it was inconsistent with the implied constitutional freedom of political communication.

South Australia has appealed to the High Court to have the decision overturned, supported by the Commonwealth, NSW, Victoria, Queensland and Western Australia and the City of Adelaide.

The Human Rights Law Centre is intervening on the side of the preachers.

In its submissions, South Australia argues that the bylaw was ''reasonably appropriate and adapted to serve a legitimate end'', consistent with a landmark High Court ruling in February which found that parole conditions imposed on a prisoner that prevented him from speaking to the media were valid.

South Australia argues the bylaw seeks to balance the many and varied competing interests of road users in the area, including pedestrians, retailers, motorists, service providers and buskers. It argues the council's permit system facilitates the expression of a diverse range of voices.

''The bylaw provides a mechanism by which all may be heard... the implied freedom is not necessarily best served by an absence of all regulation,'' it says.

It argues the degree to which the bylaw impairs the system of representative and responsible government is likely to be minimal, given other kinds of political communication are available.

In its submissions, the Human Rights Law Centre argues the bylaw ''unreasonably burdens political communications'' because the council could achieve its aims by other means that were less burdensome.

It cites exemptions in the bylaw for candidates during elections as evidence that the aim of the bylaw can be achieved without restricting political speech.

The centre argues laws from elsewhere in Australia and overseas pursue similar objectives to those pursued by the City of Adelaide while having a lesser impact on political communication.

For example, it says political communication could be ''carved out'' from the scope of the ban, a permit could only be required for a protest of a certain size, or a presumption could be created that a permit should be given unless certain circumstances apply.

Anna Brown, the Human Rights Law Centre's director of advocacy and strategic litigation, said the centre was seeking to intervene in the case because ''it will have broad ramifications for representative democracy and the right of all Australians to gather in groups and express their views and participate in the democratic process''.

The Supreme Court of South Australia's ruling on the Corneloup case was one the centre relied upon in a Federal Court challenge to a Melbourne bylaw that was used to remove Occupy Melbourne protesters from the City Square and Treasury Gardens last year. Justice Tony North has reserved his decision in that case.

Ms Brown conceded some would consider the Corneloups' preaching ''homophobic and pretty offensive''.

''The point we would make is that the Human Rights Law Centre is seeking to intervene because of the importance of this case for the right of all individuals to freely express their views and conduct peaceful protests, even if those views are rude or offensive.''