Park rangers are set to close off roads in the catchment reserve to the public in coming weeks as part of an effort to crack down on hoon and criminal behaviour, including the dumping and burning of stolen cars.

"It's not just kids on mountain bikes," Mr Daines said.

Rangers were concerned bushfires started by car fires would damage the land around the Cotter reservoir, contaminating Canberra's second-largest supply of clean drinking water.

It comes as the government also formally banned camping, off-road driving and even metal-detecting from the area on Saturday.

The changes to vehicle access to the reserve were recommended last year by ACT environment commissioner Kate Auty in her report on the Cotter, to stop stolen cars being dumped and torched in the reserve.

Her recommendations suggested larger closures but Mr Daines said firefighters still needed access to battle any potential blazes in the reserve.

Mr Daines said the new closures represented only about two to three per cent of the Cotter reserve.

"At the moment there's two dead ends in the southern part of the catchment that have been hot spots for anti-social activity," Mr Daines said.

"This will just move them back to the main road. It won't limit people's access to the catchment.

He pointed to December's Pierces Creek fire, a 200 hectare blaze started by a stolen car being set alight seven kilometres from Tuggeranong.

"If the wind had been blowing the other way, and it had gone into the catchment, the water would be unusable for another couple of decades," Mr Daines said.

The 6000 hectare reserve has a nearly 300 kilometre network of unsealed roads. Mr Daines hoped pushing people out of the more secluded spots would allow a "passive surveillance" by the community on the more frequently used roads.

Mr Daines said ACT Policing were also running night patrols with parks staff to further protect the catchment.

But when he's not dealing with hoons, Mr Daines is dealing with massive erosion gullies in the Cotter.

Sitting to Canberra's south-west, the Cotter is a recovering landscape that was once commercial pine forest before being destroyed by the 2003 bushfires.

Damaged pine doesn't hold the soil around it together, so heavy rain and floods since the blaze have cut large gullies into the earth.

One gully is about three metres wide at its widest point and nearly 10 metres high at its tallest, and it's only getting bigger.

"We actually call it the 'Grand Canyon'," Mr Daines said.

Mr Daines said there were about 30 of those gullies across the park, all of varying sizes, with the Grand Canyon one of the worst.

But because of limited parks resources, staff could only fill one gully a year.

Filling in the Grand Canyon would cost up to $150,000, requiring earth moving machines and staff maintenance.

Mr Daines said $1 million per year would be enough to close all the gullies within ten years.

Professor Auty also recommended the water levy charged to Canberra households be used to fund management of the Cotter.

The ACT government recently rejected tying the levy to management funding, instead putting it into consolidated revenue and assigning it when needed.

The levy, collected by Icon Water, earns the government about $30 million a year.