The 54-year-old former right-hand batter has fallen ill in recent days, with Nine Newspapers reporting he is in an induced coma amid a bout of meningitis.
"Lots of love and prayers sending @damienmartyn way. Keep strong and fighting legend. Love to the family," former Test teammate Darren Lehmann wrote on Twitter.
Speaking on 6PR radio, former Test bowler Rodney Hogg called Martyn's hospitalisation "shocking news".
"We wish him all the best," the former Australian paceman said.
Veteran sports broadcaster Jim Wilson was among others to send Martyn their best on social media.
Martyn's seemingly effortless stroke play was his calling card, averaging 46.37 with the bat across 67 Tests for Australia.
Born in Darwin, the right-hander earned a Test debut at 21 replacing the late Dean Jones in the 1992/93 home series against West Indies and was Western Australia's captain at 23.
His high score of 165 came against New Zealand in 2005, one of 13 Test centuries Martyn made in the baggy green.
Martyn played his final Test at Adelaide Oval in the 2006/07 Ashes series before dabbling in commentary.
Martyn played 208 ODIs, averaging 40.8.
He was part of Australia's squad that won the 2003 World Cup, scoring an unbeaten 88 in the final against India.
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