Spark has lost $1 billion in market value as investors punished the company for its poor results.
Its share price closed down 19 percent to $2.38, valuing the company at $4.39b, down from $5.4b at market open on Friday.
Spark's half-year profit fell more than three-quarters to $35 million, with revenue easing to $1.9b on the back of weaker mobile, IT and landline services.
Excluding one-offs, the company's underlying profit was 15 percent below analyst estimates, and operating expenses were higher than expected.
Spark also further downgraded its full-year earnings forecast range from $1.12b - $1.18b to $1.04b - $1.1b.
Earlier in the day, Spark's chair Justine Smyth referred to its October market update, saying the company was experiencing "one of the longest and deepest recessionary periods in recent history".
"Since that time, we have seen no improvement in these conditions, and while there has been movement on monetary policy, this is yet to flow through to any meaningful change in consumer or business spending," Smyth said.
Chief executive Jolie Hodson said it was focused on improving mobile - where it remained the market leader - reducing costs, and data centres.
"Mobile is central to our business, and we remain the market leader by some distance. This is not something we take for granted, and we have a strong pipeline of new products and campaigns that are proving popular with our customers," Hodson said.
"In our Enterprise and Government division, around 80 percent of the connection decline we have seen comes from mobile fleets shrinking as customers reduce headcounts and deliver cost efficiencies, as opposed to losing business to competitors.
