Hi Jonnie,
I hope you are not suggesting financially cutting your children off because their mum took them to Australia!! I am unsure as to what differnence the country of residence has to whether the children in question need to be funded...!!! Maintenance is a payment made by the non-resident parent to maintain (hints in the name) their children.
I would in answer to your question, presume that the maintenance agreement still stands as it is an agreement between yourself and children's mother.
If this is a situation where the problem is actually that she moved without warning, that is a custody and guardianship issue and withholding maintenance as a form of punishment is silly. If you want to pursue custody and access to your children, that is different but your children still need money to live whether here or in Oz.
I would look into your custodial rights if this is the case.