A method for monitoring a condition in at least one of a plurality of casing annuli in a subsea hydrocarbon production system having a wellhead housing mounted at the upper end of a well bore and a number of concentric well casings extending from the wellhead housing through the well bore, including an innermost casing through which a hydrocarbon fluid is produced. The plurality of casing annuli are defined between successive ones of the wellhead housing and the well casings. The method involves sensing at least a first condition at a first location in a first casing annulus, generating a first data signal at the first location which is indicative of the first condition, transmitting the first data signal from the first location to a second location either outside the wellhead housing or inside the innermost casing, receiving the first data signal at the second location; and transmitting the first data signal from the second location to a remote location at which the first condition may be monitored. The first data signal is transmitted from the first location to the second location using near-field magnetic induction (NFM) communications signals.