Penny Galanopoulos, 33, was found dead after a fire at her home on Main Street at Mordialloc in Melbourne's southeast about 5.20am on January 10, 2012.
Forensic testing revealed she was murdered before the blaze was lit.
In the same year, detectives at Mordialloc Police Station received a handwritten letter, which contained information about the fire and Ms Galanopoulos's murder, Victoria Police said.
Investigators at the time believed the information was credible and the letter's author held the key to solving the case.
However, the author never came forward despite repeated appeals from police.
Detectives on Monday announced a $1 million reward for information leading them to the people responsible for Ms Galanopoulos's murder.
Homicide squad Detective Inspector Dean Thomas said her family deserved answers and police still wanted the letter's author to come forward.
"This is very much still an active investigation and we certainly believe there are a number of people who are in a position to assist us with this investigation," Det Insp Thomas said.
"It is our belief that a number of people were potentially involved in Penny's murder and have knowledge of what happened that night.
"There is the possibility that those responsible for her death may have spoken to someone about Penny and today's reward announcement may lead to them finally coming forward with information."
The reward will be paid at Chief Commissioner Shane Patton's discretion and Director of Public Prosecutions Kerri Judd KC will consider granting people who hand over information indemnity from prosecution.
Police urged anyone with information to come forward.
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