NBN Co issued the warning on Monday after its customer research showed nine in every 10 home wi-fi set-ups had room for improvement and more than half of households did not take action until something went wrong.
The warning comes months after more than 2.7 million households received an internet speed boost through the company's upgrade program, and after thousands of homes and businesses connected with copper moved to faster fibre technology.
Three consumer studies conducted by the NBN, including its Strategic Residential Survey of more than 12,000 users, found most home internet set-ups were not delivering their potential speeds (92 per cent).
In more than half of cases, wi-fi routers were more than two years old, and almost two in three cases (59 per cent) had routers in poor locations within the home.
Common errors included operating a router away from the home's highest internet usage (47 per cent) and failing to keep it free of obstructions (19 per cent).
Seemingly innocuous objects could slow household wi-fi, NBN Co spokesman Doug MacDougall said, including brick walls, wireless appliances and water storage.
"Some of the mistakes we see are putting in near something that stops the signal, things like mirrors, furniture, solid walls," he told AAP.
"Even fish tanks and other stored liquids can absorb a wi-fi signal."
Many broadband users did not realise they were getting slower download speeds than they paid for, Mr MacDougall said, and more than half (53 per cent) did not seek to make changes until they noticed a slowdown.
Simply changing the location of their wi-fi router could improve its performance without additional spending, he said, and make more of connection upgrades.
"In April this year, 44,000 people upgraded to full fibre," he said.
"We want those customers to make sure they're getting the best bang for their buck, and the best performance and value out of their connection."
Underperforming wi-fi networks were responsible for one in every four calls to Rent-A-Grandson, according to founder Alex Stephens, and outdated modems and routers often caused them.
"Once it's all set up right, it's almost set and forget or set and forget until an issue arises," he said.
"If the fridge is keeping temperature, that's all you need, whereas with your wi-fi, it can still be on and working but not working to its full potential or even close."
Broadband users should regularly test their wi-fi speed, contact their internet provider about hardware upgrades, Mr Stephens said, and consider installing mesh wi-fi devices if household furnishings and walls proved a barrier.
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