A Christian lobby group has called on the federal government to filter out online pornography for Australian internet users.

The Australian Family Association wants a mandatory internet filter, requiring households to opt-in if they want access to adult material.

In 2013, the Coalition released a policy document backing the idea of an internet pornography filter.

Malcolm Turnbull, who was Shadow Communications Minister at the time, quickly backtracked by saying the document was "poorly worded and incorrect" and reaffirming his opposition to internet filtering.

Government emails obtained under Freedom of Information by Crikey have shown that the government is still sensitive about internet filtering.

The documents show that in 2015, the Australian Family Association wrote to government eSafety Commissioner Alastair MacGibbon, calling for an internet filter.

Emails from Mr MacGibbon to Communications Minister Mitch Fifield reportedly refer to "sensitivities around the concept of filtering for pornography".