Dumped Wallabies star Israel Folau has taken aim at transgender children and gays in his latest church sermon.

The 30-year-old said allowing children to undergo a sex change was giving in to the 'devil' during an explosive sermon at The Truth of Jesus Christ Church in Kenthurst in Sydney's north-west on Sunday.

He said the government was allowing children, 'basically 16 years old or younger', to go through treatment despite 'not even knowing what they are doing'.

The sermon comes weeks after Rugby Australia terminated his $4 million contract for a post on his Instagram which said unrepentant homosexuals would go to hell.

The 30-year-old (left with wife Maria) said allowing children to undergo a sex change was 'what the devil is trying to do' at The Truth of Jesus Christ Church

'You see in today's youths and everything, they are allowing young kids in primary school to be able to have the permission to change their gender if they want to by taking away the permission of their parents,' Folau said at the pulpit of his church.

'Now they are trying to take control as a government to make those decisions for young kids who are basically 16 years old or younger, they don't even know what they are doing.

'This is what the devil is trying to do, to instill into the government, into this world, into society, and it is slowly happening.'

The sermon was partly in response to an event in Melbourne Folau had recently attended where a person spoke about gender fluidity.

He went on to tell the congregation that born-again Christians should be bold enough to 'profess' Jesus Christ in their workplace without fear of persecution.

'True believers in Christ, are we going to follow through and profess him wherever we go,' he said.

'Are we too scared because we might be cast out by our workplace or cast out of somewhere because we're not liked or loved by those around us and don't believe the same thing we do?

'You might be the only born-again Christian in that workplace, you might feel a bit awkward with your co-workers because they are in the world and you're not.

'We should feel blessed … because God has called us.'

He said the government was allowing children, 'basically 16 years old or younger', to go through treatment but 'don't even know what they are doing'

'This is what the devil is trying to do, to instill into the government, into this world, into society, and it is slowly happening,' he said

Folau has lodged an unfair dismissal claim against Rugby Australia and will seek up to $10million in damages arguing his contract was unlawfully terminated because of his religion.

The Fair Work Commission and Folau's legal team have ruled to resolve the case on June 28 or else the dispute will lead to a hearing.

On Easter Sunday, Folau also gave a sermon at his church a week after his controversial Instagram post caused outrage.

'At some stage, each and every one of us will face our own fiery furnace, and some of us may have already faced that,' he said.

He mentioned figures in the Bible who stood up for their beliefs.

'In your workforce, if they're telling you something that will compromise your faith, this is a test of faith in which you're going to be put in a challenge, ''What are you going to do?'',

'With these guys, they were challenged and it was a matter of life and death, physical death. But they understood their treasures were stored up in heaven, not here on earth.'

46,000 people signed a #IStandWithIzzy petition, organised by the Australian Christian Lobby (ACL), after he was sacked.

The ACL gifted Folau a book with the names of his supporters written inside.