This may have been related to the rise in volatility around the turn of the year in domestic and global financial markets. Liaison with banks suggested that the tighter conditions in funding markets at the time did push banks to issue more covered bonds, since these are viewed as a good option for funding at such times.
Having said that, there is still plenty of capacity for the banks to tap their covered bond lines if they have the need. https://www.rba.gov.au/speeches/2019/sp-ag-2019-03-19.html  6/19  3/19/2019  Bonds and Benchmarks | Speeches | RBA  Graph 6  Non-Financial Corporations Over 2018, non-financial corporation spreads followed those of financial corporations, and by extension, spreads in global markets (Graph 7).