Secularism is the principle of separating Church and State. In modern times its meaning is slightly less straight forward, but still very similar.
We don’t have to worry as much about the Archbishops deciding policy for us anymore, but what we do need to be concerned about is the encroachment of religious ‘logic’ on the legislature.
This means completely dismantling the decidedly non-secular policy in Australia of funding only religious social workers as opposed to secular ones.
This means no favouritism toward religious institutions and no laws that actively encourage any religion or its spread.
This also ties into Taxation efficiency, as it means treating Religious organisations as standard non-profit organisations. They will have to follow the rules regarding all non-profits, if they can prove they are charitable they will be awarded the charity status. But no special regard will be given based on their religious status. Which obviously implies the removal of their tax free status.
Though it comes more under Equal Rights I would say it also affects marriage between homosexuals, which is absolutely acceptable within a Secular state as they’re consenting adults, the convention of marriage is a cultural one and a legal one, so its use cannot belong to a religion anymore.