Hi Nikki Hirrill,
Article 17 of the UK and Australia double taxation agreement, covers pensions and annuities, incuding a definitions of these terms. The article advises that pension payments and annuities are taxable in the country in which the individual is resident. The article make no mention of 'Trivial commutation lump sums'. This means that lumpsums are taxable in the country in which they arise.
Australia: tax treaties