The caveator, the ex husband, could voluntarily withdraw the caveat, which is not going to happen.
In Queensland, the caveat will lapse in 90 days if there have been no proceedings commenced to prove the entitlement to lodge the caveat.
If the 90 days has not elapsed, you need to serve a notice on the ex husband, a Lapsing Notice, requiring the caveator, ex husband, to start proceedings in a court to prove his interest in the property. You would arrange to serve this notice on him personally.
Once you had confirmation of service, usually by Affidavit of the person serving him, you would notify the registrar-general of Land Titles in Qld, by sending him the form 14 with a letter advising that a Lapsing Notice had been served on the caveator and a copy of the affidavit of service (only if the 90 days has not expired).
If the 90 days has expired, then you only need to complete the form above and lodge it.
I hope this is of assistance.