Noticed this promo - as per title.
If you have a current landlord policy with them, could "cancel" the current policy and renew it straight away for a further 15 months (discounted).
I assume you will be able to use the credit (for the refunded time remaining) as part payment and pay the difference (do your own due diligence).
BTW: This is Landlord Insurance - not Building Insurance.
EDIT: Check prices against others like CGU, AAMI, EBM, Honan (but compare what events each covers)
I work in Real Estate and I have found over the years that EBM and Honan insurance claim results to be better.