Internet subscribers with wireless networks are being warned to check security settings to ensure their connections are not hijacked .
Police electronic crime national manager Maarten Kleintjes told The Dominion Post it was not uncommon for people to use others' accounts.
Most did it to avoid paying for their own internet use, but some sought to download illegal material.
"They can sit in their car outside your house and download child porn. The person inside would be totally unaware," Mr Kleintjes said.
The subscriber could then be left to explain to police why such material had been accessed from their network -- although it was often possible to trace the downloads back to the offender.
Mr Kleintjes said anyone caught gaining unauthorised access to a network could face up to seven years in jail.
