The 35-year old defender, who turns 36 on the weekend, joined the Championship club in late February on a deal until the end of the season.
Sources close to Vicarage Road told SBS that Neill, who has 96 international caps, has a 'pay-per-play arrangement' with the Hornets meaning he is basically funding his stay in Hertfordshire.
Neill recently lost his spot in the Australian squad and is determined to force his way back in to Ange Postecoglou's plans so he can be involved in a third World Cup finals campaign.
The former Millwall, Blackburn Rovers, West Ham United, Everton and Galatasaray man was on the bench for Watford's 4-0 victory over Blackpool on the weekend as he seeks regular playing time under Giuseppe Sannino.
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