The writ is extraordinary discretionary jurisdiction. The photo copy of the documents attached to the petition will be generally attested by the petitioner's counsel. As such, on the face of it, there is no primary or secondary evidence as far as writ petition is concerned. But the existence of primary document, at any point of time, must be within the knowledge of the advocate, who is attesting the document. After all, it is the court's power to believe or not to believe the document.
Do you mean in your case that secondary evidence is a photostat copy of the original, in which case the same cannot be admitted. Secondly you can take a public copy of the lost document and place it before the court, convince it and will be admitted. In this regard you are directed to Secs. 64 on wards of Evidence ACt.