Failing to pay the final instalment for a purchased off-plan property

I've signed a unit reservation form back in September 2016 for an off-plan project.

At that time, it was 40% completed. I paid until mid of 2017 10% of the value of the property, as the 90% was set to be after the handover.

Now, the handover started and I can't continue to pay any further amount. I didn't sign the Sale and Purchase Agreement (SPA).

What is the legal advice here?

Thanks