Hi,
This answer can have numerous outcomes. Article 17(1) advises that pensions including lump sums and annuities, are taxable only in the country you are a resident of.
UK/ Australia Double Taxation Convention
The elimination of double taxation at article 22(2), states that if you are taxable in the UK and have already paid tax in Australia, you can claim a credit for the tax paid in Australia against the UK tax
If you are tax resident in Australia, in the tax year that the pension lump sum is paid and not the UK, then it is taxable in australia.
If you are tax resident in the UK, in the tax year that the pension lump sum is paid and not the UK, then it is taxable in the UK.
If you are tax resident in Australia and the UK, in the tax year that the pension lump sum is paid then you can claim a tax credit in the UK against the tax paid in Australia.
Thank you.