Prisons in Australia have seen a spike in drugs intercepted in mail since the coronavirus outbreak saw visits suspended, with substances concealed in letters, cards and even a child's painting, officials said Sunday.

Corrective Services NSW, which operates prisons in Australia's most populous state of New South Wales, said contraband had been seized from 135 mail items in April -- compared to a monthly average of just 22 in 2019.

That followed the suspension of visits to inmates in 38 prisons across the state in mid-March because of the coronavirus outbreak.

Contraband confiscated from the mail of inmates in NSW prisons, shown in this photo released on May 17
Contraband confiscated from the mail of inmates in NSW prisons, shown in this photo released on May 17 CORRECTIVE SERVICES NSW / -

Earlier this month, prison officers found the opioid buprenorphine concealed within a child's painting, with strips of the drug pasted onto the paper and covered by layers of paint.

"Staff became suspicious after noticing the painting of the sun had an unusual texture," Corrective Services NSW said in a statement.

Drugs have also been found taped into greeting cards and envelopes, as well as thrown over prison fences in tennis balls and drink bottles as prisons step up their searches during the pandemic shutdown.

There have been no known coronavirus outbreaks in Australian prisons. The country has recorded just over 7,000 cases and 98 deaths from the virus.