A father and son are among three people accused of helping a man take his own life, with about 20 more deaths under investigation.

The family members - aged 80 and 53 - along with an 81-year-old woman have been charged over the alleged assisted suicide of a Gold Coast man in April.

The elderly duo supported the 53-year-old at an end-of-life business which provided the victim a veterinary euthanasia drug sourced under false pretences, police allege.

The investigation has been widened to include other deaths dating back to 2021, with "around or greater than 20" to be probed including some in Victoria and NSW.

"The alleged offender is deliberately targeting vulnerable people, taking advantage of them in their most desperate moments," Detective Inspector Mark Mooney said of the 53-year-old

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