Street prostitution has become ever more visible in Norwegian cities in recent years, with prostitute support organisations estimating the country of just 4.6 million people counts as many as 3,000 sex workers.
The new law, which is modelled on similar legislation in Sweden, aims to clean up the streets and protect the prostitutes by outlawing the buying of sex, but not the sale.
Procuring, or "pimping", and human trafficking are already illegal.
Norway will go even further than its Scandinavian neighbour however, making it illegal for Norwegian citizens and residents to purchase sexual favours even abroad, although Aas-Hansen insists catching johns in foreign countries "is not a priority for Norwegian police."
Prostitutes' customers could be slapped with fines proportionate to their revenues, be sentenced to up to six months in prison, or both.
In extreme cases, especially when the person providing sexual services is a minor, the prison term can stretch up to three years.
Norwegian media has reported that street prostitution has dropped considerably in the run-up to the introduction of the new law, but Bjoerg Norli of the Pro prostitute support centre insists the decline is an illusion brought on by plunging winter temperatures.
"The women are waiting to see what will happen. They have not decided yet whether they will leave or stop selling sex or continue and establish indoors," she told AFP.
When the centre-left coalition government said in July 2007 that it was planning to draft the law, it drew protests from support groups like Pro who claimed it would make sex workers more reliant on pimps to get customers and would force them to work in more secluded places, making them more vulnerable to rape and attack by clients.
"That's the risk ... Some of us feared the situation of the prostitutes would get worse," Aas-Hansen acknowledged, but pointed out that after much debate the law had been equipped with mechanisms aimed at helping the prostitutes.
"That's why our plan includes help for the prostitutes."
Under the new law, prostitutes will have access to free schooling, police assistance and detoxification treatment for those who are addicted to drugs or alcohol.
Norli was not convinced however, pointing out while those wishing to quit the profession would surely receive plenty of help the new law would make life much more difficult for sex workers who felt they had no alternative.
"I'm sure it's going to be a lot about helping the women who would like to quit and get extra schooling or assistance to get a job," she told AFP.
But "our experience is not that a lot of women are leaving prostitution, but that some of them have left (the country) because of the law," she said.
The Pro centre suspects many of the Nigerian women, who make up one of the largest groups of foreign prostitutes in the Scandinavian country, "will leave Norway and continue prostitution elsewhere in Europe, which is just moving the problem somewhere else."
And for those who stay behind, "everything is going to be more difficult, the streets will be more difficult," Norli insisted.
"Our greatest concern are the Norwegian women who are drug dependent. They really don't have an alternative. They won't have any income source with the new law," she said.
The Norwegian law is modelled on a law adopted in Sweden in 1999 that has been hailed by police and even some sex worker support groups as highly effective at reducing prostitution.
Finland introduced a similar law in 2006, while Scotland also criminalised the purchase of sex in 2007.