The southern coast of Australia is used to huge seas and nature has adjusted accordingly. On the East Coast however, truly massive seas may only occur once every few decades. Most are associated with Tropical Cyclones or East Coast lows. As cyclones move southwards out of tropical waters they can interact with high pressure further southwards. The resultant isobar compression can cause strong to gale force winds to affect a much wider expanse of ocean than was the case when the cyclone was in tropical waters.
Huge seas can result.
I'm sorry I can't be more specific. I think it was Cyclone Colin. This cyclone took a track down into the Tasman Sea. Rather than curving off SE towards the north island of New Zealand, the cyclone ran southwards between Australia and New Zealand. The resultant seas were huge.