You can move if you don't have any current orders in place, but your ex will be within his right to seek a recovery order to bring the child back to Darwin.
Remember, when it comes to family law, you have no rights. Only your child does. That right is to know, spend time with and be cared for by both parents. If it were to end up in court, the court would make orders that only consider the child's best interests, not yours, and you would need to show why it's in the child's best interests to be moved away from his dad to accommodate your employment opportunity.
I would suggest going to mediation with your ex-husband to discuss care arrangements that put the child's best interests first. Perhaps alternate weekends with dad and an exchange point half way?
Hope this helps.