One dog is dead and another is fighting for life following a suspected dog-baiting incident being investigated by police in Melbourne's north-west.

Ian Davies’ beloved two-year-old Belgian Shepherd, Chance, remains critically ill after consuming an unknown poisonous substance at his home in Underhill Court in Sunbury late Sunday night. ­

Rua died after ingesting an unknown substance.

"We've been told his toxins levels are still through the roof," Mr Davies told radio station 3AW.

"He's critically ill ... he's an absolutely beautiful dog, it's just been a horrendous few days."