He spotted "the remnants of brown leatherette sofas and and ruins of a flat-screen TV set" - on Redcombe Lane.
Our informant described it as "someone's grot" that had already been there for a couple of days.
It had been dumped outside a disused gateway at the Vale Academy's Redcombe Lane site.
"The old school buildings are being demolished, but behind locked gates, contractors' rubbish is stacked in neat piles, ready for disposal," he said.
Although PCSOs attended to view the dumped rubbish, a spokeswoman for the Humberside force said this was a matter for North Lincolnshire Council.
"Those who dump their rubbish for others to pick up are acting illegally and irresponsibly.
"There are many ways you can dispose of large items – the council offers a bulky item collection service, free of charge for up to three items in one collection each year.
"You can also take them to any of our eight household waste recycling centres – a permit may be required.
“The council will, on this occasion, remove the rubbish and urges anyone who witnesses flytipping to come forward.
"You can report it online at www.northlincs.gov.uk
"Help the council catch those responsible by reporting any incidents.”