Your UK will does not immediately become invalid (as far as the UK is concerned, although you should confirm this with a UK solicitor if important) when you move to Australia.
However, if you are settling permanently in Australia you should normally replace it with a local will.
- If your assets are in Australia, then probate will need to be done in Australia. Having your "out of jurisdiction" UK will admitted for probate in Australia will - as a minimum - be slower and more costly, and whether it can be done at all is dependent on the law of the Australian state/territory.
- Making an Australian will helps with losing your UK domicile, which normally you want to do for Inheritance Tax purposes.
- If you want to make provision for guardianship of children, an Australian will may be the best option, again depending on local laws.
Should you have both a UK and Australian will?
- The problem with having two wills is ensuring that one does contradict the other, also may conflict with the law in either jurisdiction
- If you still have significant assets in the UK it may still be better to have a single will made in Australia.
- This will would then need to be probated in the UK on an "out of jurisdiction" basis,
- There may be an option to register an Australian will with the UK courts, in advance, to speed up this process
- Discuss with a UK solicitor who specialises in wills/estates
- Longer term, it is one of the factors you should consider when you decide whether or not to leave assets in the UK.