The 22-year-old shooter will link up with former Australian international Megan Dehn, former Silver Fern Daneka Wipiiti and Julianna Naoupu in the shooting circle.
Her transfer south completes the Steel's playing puzzle for 2010.
Steel chief executive Julie Patterson was pleased with the final squad and said Topia would be a valuable addition.
"We felt we needed an experienced shooter to come on board with us," she said.
"With Daneka being a first-time mum, we felt it was necessary for us to have some experience in our backup team," Patterson said.
"Our shooting end is looking pretty strong now, which is really good."
Topia has played for the Mystics for the past two seasons and played for the Canterbury Flames in the now-defunct National Bank Cup.
In 2007, she was selected in a Silver Ferns training squad and has been selected in New Zealand A and New Zealand under-21 teams.
In seven games for the Mystics last season she scored 76 goals from 97 attempts.
The Steel also confirmed Robyn Broughton as the coach for next year.
Broughton led the Steel to the play-offs last season and coached the Southern Sting to seven titles in 10 years.
