A friend keeps telling me aussies and brits can't use that border anymore? We are banned from entering cambodia. Is this true?

All borders are closed. You can only get out by flying back to your home country.

Dont know how long it will last.

"The Cambodia-Laos, Cambodia-Thailand and Cambodia-Vietnam land borders are closed."

https://www.gov.uk/foreign-travel-advice/cambodia/coronavirus

All land crossings between Thailand and neighbouring countries are closed to foreigners. In the unlikely event that you managed to leave, in most cases you would not be allowed to return to Thailand. Excluding special categories like diplomats, the only foreigners allowed into Thailand at present are those with work permits. According to the letter of the published regulations, even foreigners with permanent residence who happen to be outside the country cannot return (though this is almost certainly an error in the drafting of the regulations). 

thank you for info

All closed. Even when poi pet was open it was a troublesome crossing for a border hop.