Black bags will be checked at the kerbside in Swansea for plastic, paper and glass under controversial new measures.
Council officials have said bin bags will not be opened, but shaken to listen for recyclable materials.
Those who still refuse to sort their waste will be sent letters before facing a fine.
Some residents say the measure is snooping but Swansea Council said it helped increase recycling rates.
Under Welsh Government targets, councils in Wales have to meet a 64% recycling rate by 2019/20.
In Swansea, the projected rate for 2018/19 is around 63%.
The new checks were approved by council cabinet members at a meeting on Thursday.
According to the Local Democracy Reporting Service, council workers will record addresses with a significant amount of recyclables before knocking on the door or leaving a letter to encourage people to recycle more.
That process will be repeated twice more before a fixed penalty notice is issued.
Councillors say people making the odd mistake would not be affected.
Cabinet members approved the plans at a meeting on Thursday.
Environment cabinet member Mark Thomas said the council would employ "extra officers to do the monitoring" - but how much money and how many new posts would be created had not yet been decided.
News of the scheme received a mixed reaction from residents in Swansea.
Katie McLaren, of Gorseinon, said: "It's an invasion of privacy."
And city centre resident Pete Sanders added: "The amount we pay in council tax should surely cover the cost of a sorting plant like other countries have."
However, father-of-four Andrew Evans said: "There's no excuse not to recycle. Why should some people be allowed to get away with chucking stuff in black bags when the rest of us recycle?"
No date for the roll-out has been given yet, but officials say it will happen after a publicity campaign.
The Welsh Local Government Association said Rhondda Cynon Taf council had a similar measure in place.
A spokesman added: "Local government in Wales is exploring an array of different options to improve recycling rates, and which would support the extensive education and engagement work being undertaken by councils on recycling and the environment."