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Nineteen-year-old Erwood is in her debut with the ANZ Championship side and has yet to take the court, instead playing a leading role for South in the second-tier national league.
While it seems likely Noeline Taurua will stick with first-choice duo Jane Watson and Te Huinga Selby-Rickit at the defensive end for the match, the Steel coach said she would have full confidence in thrusting the youngster into the action from the bench against the Melbourne-based side if she had to.
"She will take the court if need be, and I've got no worries in putting her in.
"She's well prepared to go out there.''
Taurua did not want to elaborate on the illness but said Purvis should be back training with the side next week.
She will sit on the bench for the team today but will play no part in the game.
"It's a slow process to get her back to where she needs to be.
"We're lucky she can still be a part of the team and, while she can't contribute on the court, she can still make a huge contribution in regards to her defensive nous.''
A win today would guarantee the Steel top spot in the transtasman competition and ensure home court advantage for all of its playoff matches.
The Vixens also have plenty to play for, as they fight for the final playoff spot in the Australian conference with the West Coast Fever.
The Vixens travel to Wellington to play the Pulse in the final round next week and Taurua was hoping its travel schedule would give the home side an advantage.
"The Australian conference is very similar to the New Zealand conference with the competition for that third spot.
"Like all the teams [in contention], they'll be fighting it out for that last position.
"They've got a tough road in, playing us down here in Invercargill then flying to Wellington to take on the Pulse, and also coming up against the New Zealand style.
"We want to be able to take the lead early and take advantage of the position that they're in.''
While the Steel is unbeaten in 11 matches, and is looking for its 10th win in a row, it will face one of its toughest tests of the season today against a combative and battle-hardened Vixens outfit.
History is in the home side's favour, however, as the 2014 champion has won only once in four attempts in the deep South, in 2012.
The last time the sides met in New Zealand was in the Vixens' victorious year, 2014, when the Steel pipped the visitors 65-64 in Invercargill.
The visiting side has strengths all over the court and the Steel will need to put in a complete defensive performance if it is to come out on top, Taurua said.
The match will again pit veteran midcourters, and captains, Wendy Frew (Steel) and Madi Robinson (Vixens) against each other in what should be another fierce battle.
English international Geva Mentor is second in the competition for deflections and sixth in intercepts, while Karyn Bailey is in the top 10 goal shooters.