Two of the most popular gadgets on the market are a device that recovers deleted text messages from mobile phones and a chemical test kit to detect traces of semen on underwear.
OzSpy's SIM Card Data Recovery Pro works by placing the SIM card into the gadget and then plugging it into a computer to download the stored information, which includes deleted text messages.
And the CheckMate 5-Minute Infidelity Test Kit works in a similar way to a home pregnancy test.
The company claims to be selling more than 100 of each device a month nationwide - more than twice the number sold six months ago.
Victoria's Privacy Commissioner Helen Versey said that while she had concerns about the increasing use of spy gadgets, there was no law banning their use by individuals.
Whenever there is technology which potentially invades the privacy of individuals, I have a concern about the potential misuse of them," theage.com.au quoted Versey, as saying.
"As with any technology, it can be used for good but is also subject to misuse," she added.
Relationships Australia Victoria Ballarat centre manager Jane Vanderstoel said counsellors had reported an increase in the number of couples using technology to check on their partner's fidelity.
"As people get more technologically savvy, they are using these devices to confirm suspicions or to gather evidence," she said.
"It's pretty normal human behaviour," she added.